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Document and Entity Information - shares	9 Months Ended
Sep. 30, 2017	Nov. 09, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Trading Symbol	ALRN
Entity Registrant Name	AILERON THERAPEUTICS INC
Entity Central Index Key	0001420565
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		14,718,787

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 13,428	$ 20,715
Investments	43,437	0
Prepaid expenses and other current assets	1,164	333
Restricted cash	88	25
Total current assets	58,117	21,073
Property and equipment, net	111	107
Restricted cash		63
Other assets	808	778
Total assets	59,036	22,021
Current liabilities:
Accounts payable	1,650	1,971
Accrued expenses and other current liabilities	3,373	2,100
Total current liabilities	5,023	4,071
Deferred rent		11
Total liabilities	5,023	4,082
Commitments and contingencies (Note 11)
Redeemable convertible preferred stock (Series A, A-1, B, C-1, C-2, D, D-1, E, E-1, E-2, E-3 and F), $0.01 par value; no shares and 151,557,293 shares authorized at September 30, 2017 and December 31, 2016, respectively; no shares and 105,631,019 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively		129,745
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value; 5,000,000 shares and no shares authorized at September 30, 2017 and December 31, 2016, respectively; no shares issued and outstanding at September 30, 2017 and December 31, 2016
Common stock, $0.001 par value; 150,000,000 shares and 143,500,000 shares authorized at September 30, 2017 and December 31, 2016, respectively; 14,718,787 and 432,413 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	15
Additional paid-in capital	184,085	2,536
Accumulative other comprehensive loss	(6)
Accumulated deficit	(130,081)	(114,342)
Total stockholders’ equity (deficit)	54,013	(111,806)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 59,036	$ 22,021

Condensed Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Redeemable convertible preferred stock, authorized (in shares)	0	151,557,293
Redeemable convertible preferred stock, issued (in shares)	0	105,631,019
Redeemable convertible preferred stock, outstanding (in shares)	0	105,631,019
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	143,500,000
Common stock, shares issued	14,718,787	432,413
Common stock, shares outstanding	14,718,787	432,413

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	3,825	2,605	9,928	7,184
General and administrative	2,601	1,627	6,039	6,537
Total operating expenses	6,426	4,232	15,967	13,721
Loss from operations	(6,426)	(4,232)	(15,967)	(13,721)
Interest and other income	167	10	228	40
Net loss	(6,259)	(4,222)	(15,739)	(13,681)
Accretion of redeemable convertible preferred stock to redemption value		(19)	(41)	(57)
Net loss attributable to common stockholders	$ (6,259)	$ (4,241)	$ (15,780)	$ (13,738)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.45)	$ (9.84)	$ (3.16)	$ (32.04)
Weighted average common shares outstanding—basic and diluted	13,939,950	430,929	4,990,535	428,770
Other comprehensive loss:
Unrealized loss on investments, net of tax of $0	$ (6)	$ (3)	$ (6)
Total other comprehensive loss	(6)	(3)	(6)
Comprehensive loss	$ (6,265)	$ (4,225)	$ (15,745)	$ (13,681)

Condensed Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - 9 months ended Sep. 30, 2017 - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2016	$ 129,745
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2016	105,631,019
Issuance of Series F redeemable convertible preferred stock, net of issuance costs of $32	$ 626
Issuance of Series F redeemable convertible preferred stock, net of issuance costs, Shares	483,501
Accretion of redeemable convertible preferred stock to redemption value	$ 41
Conversion of redeemable convertible preferred stock to common stock	$ (130,412)
Conversion of redeemable convertible preferred stock to common stock, Shares	(106,114,520)
Redeemable convertible preferred stock, ending balance, shares at Sep. 30, 2017	0
Beginning balance at Dec. 31, 2016	$ (111,806)		$ 2,536		$ (114,342)
Beginning balance, shares at Dec. 31, 2016	432,413	432,413
Accretion of redeemable convertible preferred stock to redemption value	$ (41)		(41)
Conversion of redeemable convertible preferred stock to common stock	130,412	$ 11	130,401
Conversion of redeemable convertible preferred stock to common stock, Shares		10,509,774
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs	50,009	$ 4	50,005
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs, shares		3,750,000
Exercise of stock options	$ 100		100
Exercise of stock options, shares	26,600	26,600
Stock-based compensation expense	$ 1,084		1,084
Unrealized loss on investments, net of tax of $0	(6)			$ (6)
Net loss	(15,739)				(15,739)
Ending balance at Sep. 30, 2017	$ 54,013	$ 15	$ 184,085	$ (6)	$ (130,081)
Ending balance, shares at Sep. 30, 2017	14,718,787	14,718,787

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (15,739)	$ (13,681)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	85	190
Net amortization of premiums and discounts on investments	(88)	(14)
Stock-based compensation expense	1,084	519
Change in deferred rent	(11)	(29)
Loss on disposal of property and equipment		12
Write-off of deferred offering costs		1,500
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(909)	(336)
Other assets	33
Accounts payable	(233)	284
Accrued expenses and other current liabilities	1,085	774
Net cash used in operating activities	(14,693)	(10,781)
Cash flows from investing activities:
Purchases of property and equipment	(74)
Purchases of investments	(52,130)	(12,634)
Proceeds from sales or maturities of investments	8,775	11,150
Changes in restricted cash		(25)
Net cash used in investing activities	(43,429)	(1,509)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	471	13,183
Proceeds from exercise of stock options	100	23
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	52,313
Payments of initial public offering costs	(2,049)	(180)
Net cash provided by financing activities	50,835	13,026
Net increase (decrease) in cash and cash equivalents	(7,287)	736
Cash and cash equivalents at beginning of period	20,715	3,768
Cash and cash equivalents at end of period	13,428	4,504
Supplemental disclosure of non-cash financing activities:
Accretion of redeemable convertible preferred stock to redemption value	41	$ 57
Deferred offering costs included in accounts payable and accrued expenses	255
Conversion of convertible preferred stock into common stock upon listing of the Company's common stock on the NASDAQ	$ 130,412

Nature of the Business and Basis of Presentation	9 Months Ended
Nature of the Business and Basis of Presentation	Sep. 30, 2017
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Aileron Therapeutics, Inc. (“Aileron” or the “Company”) is a clinical-stage biopharmaceutical company that is focused on developing and commercializing a novel class of therapeutics called stapled peptides. The Company’s lead product candidate, ALRN-6924, targets the tumor suppressor p53 for the treatment of a wide variety of cancers. ALRN-6924 reactivates p53-mediated tumor suppression by targeting the two primary p53 suppressor proteins, MDMX and MDM2. ALRN-6924 was in multiple clinical trials as of September 30, 2017 and December 31, 2016. The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary governmental regulatory approval or that any approved products will be commercially viable. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its key employees and consultants. Reverse Stock Split On June 16, 2017, in connection with its initial public offering (“IPO”), the Company effected a one-for-9.937 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 6). Accordingly, all common share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and the associated adjustment of the preferred stock conversion ratios. Initial Public Offering On June 28, 2017, the Company’s registration statement on Form S-1 relating to its initial public offering of its common shares (the “IPO”) was declared effective by the Securities and Exchange Commission (“SEC”). In the IPO, which closed on July 5, 2017, the Company issued and sold 3,750,000 shares of common stock at a public offering price of $15.00 per share for net proceeds of $50.0 million after deducting underwriting discounts and commissions of $3.9 million and estimated offering expenses of $2.3 million. Upon the closing of the IPO, all 106,114,520 shares of redeemable convertible preferred stock then outstanding converted into an aggregate of 10,509,774 common shares. Liquidity In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosures of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), management must evaluate whether there are conditions or events, considered in the aggregate, that raise substantial doubt about the company’s ability to continue as a going concern within one year after the date that the financial statements are issued. This evaluation initially does not take into consideration the potential mitigating effect of management’s plans that have not been fully implemented as of the date the financial statements are issued. When substantial doubt exists under this methodology, management evaluates whether the mitigating effect of its plans sufficiently alleviates substantial doubt about the company’s ability to continue as a going concern. The mitigating effect of management’s plans, however, is only considered if both (1) it is probable that the plans will be effectively implemented within one year after the date that the financial statements are issued, and (2) it is probable that the plans, when implemented, will mitigate the relevant conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Generally, to be considered probable of being effectively implemented, the plans must have been approved before the date that the financial statements are issued. The Company’s financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. Through September 30, 2017, the Company has funded its operations with net proceeds of $50.0 million from its IPO, $131.2 million from sales of preferred stock and $34.9 million from a collaboration agreement. As of September 30, 2017, the Company had cash, cash equivalents and investments of $56,865 million. The Company has incurred losses and negative cash flows from operations and had an accumulated deficit of $130,081 as of September 30, 2017. The Company expects to continue to generate losses for the foreseeable future. As of November 9, 2017, the date of issuance of these unaudited interim condensed financial statements, the Company expects that its cash, cash equivalents and investments of $56,865 as of September 30, 2017 will be sufficient to fund its operating expenses and capital expenditure requirements through at least the next twelve months. The future viability of the Company is dependent on its ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. To execute its business plans, the Company will need substantial funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of common stock in public offering and/or private placements, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain financing on acceptable terms or at all. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion plans or commercialization efforts, which could adversely affect its business prospects. The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual of research and development expenses and the valuation of common stock and stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Unaudited Interim Financial Information

The accompanying unaudited condensed financial statements as of September 30, 2017 and for the nine months ended September 30, 2017 and 2016 have been prepared by the Company, pursuant to the rules and regulations of the SEC for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2016 included in the Company’s prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-218474). The prospectus was filed with the SEC pursuant to Rule 424(b)(4) on June 29, 2017.

The unaudited interim condensed financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2017, the results of its operations for the three and nine months ended September 30, 2017 and 2016 and its cash flows for the nine months ended September 30, 2017 and 2016. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2017 and 2016 are unaudited. The results for the nine months ended September 30, 2017 are not necessarily indicative of results to be expected for the year ending December 31, 2017, any other interim periods, or any future year or period. The accompanying balance sheet as of December 31, 2016 has been derived from the Company’s audited financial statements for the year ended December 31, 2016 previously filed with the SEC.

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, corporate notes and commercial paper, are stated at fair value.

Restricted Cash

As of September 30, 2017, current restricted cash of $88 consisted of $25 of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards and $63 of cash deposited in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. As of December 31, 2016, current restricted cash consisted of $25 of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards and non-current restricted cash consisted of $63 of cash deposited in a separate restricted bank account as a security deposit for the lease of the Company’s facilities.

Investments

The Company classifies its available-for-sale investments as current assets on the balance sheet if they mature within one year from the balance sheet date.

The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities or using other inputs that are observable or can be corroborated by observable market data. Unrealized gains and losses on available-for-sale securities are reported as accumulated other comprehensive income (loss), which is a separate component of stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the statements of operations and comprehensive loss.

The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the statements of operations and comprehensive loss. No such adjustments were necessary during the periods presented.

Fair Value Measurements

Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable.

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities.

Deferred Offering Costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should the planned equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the statement of operations and comprehensive loss.

The Company’s IPO was completed in July 2017 and deferred offering costs of $2,304 were recorded as a reduction to stockholder's equity. As of September 30, 2017 and December 31, 2016, the Company did not have any deferred offering costs recorded. As of December 31, 2015, the Company had recorded $1,500 of deferred offering costs in contemplation of a probable 2016 equity financing. The Company determined in June 2016 that the equity financing was no longer probable of being consummated and, at that time, recorded general and administrative expense of $1,500 in the statement of operations and comprehensive loss to write-off the deferred offering costs that had been capitalized.

Net Income (Loss) per Share

The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed.

Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting income (loss) per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and shares of redeemable convertible preferred stock are considered potential dilutive common shares.

The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but contractually did not require the holders of such stock to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

Recently Issued Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle, and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delays the effective date of ASU 2014-09 such that the standard is effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption of the standard is permitted for annual periods beginning after December 15, 2016. In March 2016, the FASB issued ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (“ASU 2016-08”), which further clarifies the implementation guidance on principal versus agent considerations in ASU 2014-09. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing (“ASU 2016-10”), clarifying the implementation guidance on identifying performance obligations and licensing. Specifically, the amendments in this update reduce the cost and complexity of identifying promised goods or services and improve the guidance for determining whether promises are separately identifiable. The amendments in this update also provide implementation guidance on determining whether an entity’s promise to grant a license provides a customer with either a right to use the entity’s intellectual property (which is satisfied at a point in time) or a right to access the entity’s intellectual property (which is satisfied over time). In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients (“ASU 2016-12”), which clarifies the objective of the collectability criterion, presentation of taxes collected from customers, non-cash consideration, contract modifications at transition, completed contracts at transition and how guidance in ASU 2014-09 is retrospectively applied. ASU 2016-08, ASU 2016-10 and ASU 2016-12 have the same effective dates and transition requirements as ASU 2014-09. The adoption of these standards is not expected to have an impact on the Company’s financial position, results of operations or cash flows as the Company does not currently have any revenue-generating arrangements.

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities. The ASU affects the accounting for equity investments, financial liabilities under the fair value option and the presentation and disclosure requirements for financial instruments. In addition, it includes a clarification related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. The standard is effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Entities can early adopt the provision to record changes in the fair value of financial liabilities under the fair value option resulting from instrument-specific credit risk separately in other comprehensive income. Early adoption of this provision can be elected for all financial statements for annual and interim periods that have not yet been issued (for public business entities) or that have not yet been made available for issuance. The adoption of this standard is not expected to have a material impact on the Company’s financial position, results of operations or cash flows.

In February 2016, the FASB issued ASU No. 2016-02, Leases, (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees or lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months, regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 (Accounting Standards Codification (“ASC”) Topic 842) supersedes the previous leases standard, ASC 840, Leases. The standard is effective for public entities for annual periods beginning after December 15, 2018 and for interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-02 will have on its financial statements.

In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (“ASU 2016-09”), which amends ASC Topic 718, Compensation—Stock Compensation (“ASC Topic 718”). The new standard identifies areas for simplification involving several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, an option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, as well as certain classifications on the statements of cash flows. The standard is effective for annual periods beginning after December 15, 2016 and for interim periods within those fiscal years. The Company adopted ASU 2016-09 on the required effective date of January 1, 2017. The Company elected to maintain its existing policy to estimate forfeitures when determining periodic stock-based compensation expense. The adoption of the other provisions of ASU 2016-09 had no impact on the Company’s financial position, results of operations or cash flows.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). This guidance addresses the presentation and classification of certain cash receipts and cash payments in the statement of cash flows. The standard is effective for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-15 will have on its financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”). The new standard requires restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the total beginning and ending amounts for the periods shown on the statement of cash flows. The new standard is effective for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-18 will have on its financial statements.

In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (“ASU 2017-09”), Scope of Modification Accounting which amends ASC Topic 718. This new standard clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for all companies for annual periods beginning on or after December 15, 2017 and for interim periods within those fiscal years. The Company is currently evaluating the potential impact that the adoption of ASU 2017-09 will have on its financial statements.

Fair Value of Financial Assets

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets

3. Fair Value of Financial Assets

The following tables present information about the Company’s assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values:

	Fair Value Measurements as of September 30, 2017 using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	10,321	$	—	$	—	$	10,321
Corporate notes		—		1,000		—		1,000
Commercial paper		—		1,998		—		1,998
Investments:
Corporate notes		—		24,910		—		24,910
Commercial paper		—		18,527		—		18,527
	$	10,321	$	46,435	$	—	$	56,756

	Fair Value Measurements as of December 31, 2016 using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	20,016	$	—	$	—	$	20,016
	$	20,016	$	—	$	—	$	20,016

As of September 30, 2017, the Company’s cash equivalents and investments were invested in money market funds, corporate notes and commercial paper and were valued based on Level 1 and Level 2 inputs. As of December 31, 2016, the Company’s cash equivalents were invested in money market funds and were valued based on Level 1 inputs. In determining the fair value of its corporate notes and commercial paper at each date presented above, the Company relied on quoted prices for similar securities in active markets or using other inputs that are observable or can be corroborated by observable market data. The Company’s cash equivalents have original maturities of less than 90 days from the date of purchase. All available-for-sale investments have contractual maturities of less than one year. During the nine months ended September 30, 2017and the year ended December 31, 2016, there were no transfers between Level 1, Level 2 and Level 3.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Redeemable Convertible Preferred Stock

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Redeemable Convertible Preferred Stock

6. Redeemable Convertible Preferred Stock

As of September 30, 2017 and December 31, 2016, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue no shares and 151,557,293 shares of $0.01 par value redeemable convertible preferred stock, respectively.

The Company previously had issued Series A, Series A-1, Series B, Series C-1 and Series C-2 redeemable convertible preferred stock (collectively, the “Junior Preferred Stock”) and Series D, Series D-1, Series E, Series E-1, Series E-2, Series E-3 and Series F redeemable convertible preferred stock (collectively, the “Senior Preferred Stock”), together the “Redeemable Preferred Stock”. The Redeemable Preferred Stock is classified outside of stockholders’ equity (deficit) because the shares contain redemption features that are not solely within the control of the Company.

In January 2016, the Company issued 9,705,882 shares of Series E-1 preferred stock at a price of $1.36 per share, resulting in proceeds of $13,183, net of issuance costs of $17.

In December 2016, the Company issued 13,949,357 shares of Series F redeemable convertible preferred stock (the “Series F preferred stock”) at a price of $1.36 per share, resulting in proceeds of $18,806, net of issuance costs of $165. As part of the Series F preferred stock purchase agreement, the investors agreed to purchase an additional 8,192,477 shares of Series F preferred stock at a price of $1.36 per share upon the Company achieving specified clinical milestones (the “second tranche closing”) for an aggregate purchase price of $11,142. In February 2017, the Company amended the Series F preferred stock purchase agreement to permit the sale of up to 758,458 additional shares of Series F preferred stock. In February 2017, pursuant to the amended Series F preferred stock purchase agreement, the Company issued 483,501 shares of Series F preferred stock at a price of $1.36 per share, resulting in proceeds of $626, net of issuance costs of $32. The purchasers of Series F preferred stock in this February 2017 closing agreed to purchase an additional 274,957 shares of Series F preferred stock at a price of $1.36 per share in the second tranche closing, which increased the aggregate number of shares of Series F preferred stock to be purchased in the second tranche closing to 8,467,434 shares for an aggregate purchase price of $11,516. The Company determined that the future tranche obligations of the Series F preferred stock purchase agreement, as amended, did not meet the definition of a freestanding financial instrument because, while separately exercisable, they were not legally detachable. Further, the Company determined that the embedded future tranche obligations did not require bifurcation for accounting purposes as they are clearly and closely related to the economic characteristics and risks of the initial preferred shares and would not qualify as a derivative on a standalone basis.

In December 2016, pursuant to the Series F preferred stock purchase agreement, holders of 8,927,582 shares of Series E redeemable convertible preferred stock (the “Series E preferred stock”) that participated in the Series F preferred stock financing elected to convert their shares of Series E preferred stock into 8,927,582 shares of Series E-2 preferred stock, and holders of 16,567,108 shares of Series E-1 preferred stock that participated in the Series F preferred stock financing elected to convert their shares of Series E-1 preferred stock into 16,567,108 shares of Series E-3 redeemable convertible preferred stock (the “Series E-3 preferred stock”). Holders of Series E preferred stock and Series E-1 preferred stock that did not participate in the Series F preferred stock financing were not entitled to convert their shares into Series E-2 preferred stock and Series E-3 preferred stock, respectively. In February 2017, pursuant to the amended Series F preferred stock purchase agreement, holders of 4,411,765 shares of Series E-1 preferred stock that participated in the February 2017 closing elected to convert their shares of Series E-1 preferred stock into 4,411,765 shares of Series E-3 preferred stock.

In February 2017, the Company issued 483,501 shares of Series F preferred stock at a price of $1.36 per share, resulting in proceeds of $626, net of issuance costs of $32.

The Company determined that the conversion of shares of preferred stock that occurred in December 2016 and February 2017 represented modifications of these securities for accounting purposes; however, the modifications did not result in the recognition of a deemed dividend for accounting purposes because the modifications did not result in a transfer of value from common stockholders to preferred stockholders.

Pursuant to the terms of the amended Series F preferred stock purchase agreement, if the second tranche closing did not occur prior to the closing of the Company’s initial public offering of common stock, then, immediately prior to such closing, the purchasers of the Series F preferred stock would be required to purchase a number of shares of the Company’s common stock equal to $11,516 divided by the price per share paid by the public in the initial public offering in a concurrent private offering. This requirement to purchase shares immediately prior to the closing of the Company’s initial public offering could be waived in whole or in part by the Company’s board of directors. As of December 31, 2016, the specified clinical milestones had not been achieved and the second tranche closing had not occurred.

On June 15, 2017, the Company’s board of directors waived in whole, effective immediately prior to the closing of the Company’s IPO, the requirement of the purchasers of Series F preferred stock to purchase shares of the Company’s common stock in a concurrent private offering in connection with the Company’s initial public offering.

Upon the completion of the Company’s IPO on July 5, 2017, all shares of the Redeemable Preferred Stock converted into an aggregate of 10,509,774 shares of common stock. As of September 30, 2017, there were no shares of Redeemable Preferred Stock authorized, issued or outstanding.

Redeemable Preferred Stock consisted of the following as of:

	December 31, 2016
	Preferred Shares Authorized		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		1,250,000		1,250,000	$	1,250	$	1,250		12,579
Series A-1 preferred stock		615,384		615,384		800		800		6,192
Series B preferred stock		3,706,056		3,706,056		1,506		1,506		372,955
Series C-1 preferred stock		5,934,050		5,934,050		6,997		7,000		597,167
Series C-2 preferred stock		8,689,144		8,689,144		10,248		10,250		874,423
Series D preferred stock		34,142,865		34,142,865		40,263		40,276		3,435,932
Series D-1 preferred stock		363,636		363,636		2,000		2,000		36,594
Series E preferred stock		12,715,822		3,788,240		4,453		4,469		381,225
Series E-1 preferred stock		24,264,705		7,697,597		10,446		10,469		774,639
Series E-2 preferred stock		9,226,082		8,927,582		10,493		10,531		898,418
Series E-3 preferred stock		21,237,785		16,567,108		22,483		22,531		1,667,214
Series F preferred stock		29,411,764		13,949,357		18,806		18,971		1,403,779
		151,557,293		105,631,019	$	129,745	$	130,053		10,461,117

The holders of the Redeemable Preferred Stock had the following rights and preferences prior to conversion:

Voting Rights

The holders of the Redeemable Preferred Stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote and had the right to vote the number of shares equal to the number of whole shares of common stock into which such holders of Redeemable Preferred Stock could convert on the record date for determination of stockholders entitled to vote. In addition, holders of the Senior Preferred Stock, voting as a single class, were entitled to elect three directors of the Company. The holders of the Junior Preferred Stock, voting as a single class, were entitled to elect two directors of the Company.

Dividends

The holders of the Redeemable Preferred Stock, in order of preference, were entitled to receive noncumulative dividends when and if declared by the Company’s board of directors. The Company could not declare, pay or set aside any dividends on shares of any other class or series of capital stock of the Company unless the holders of the Redeemable Preferred Stock then outstanding first received, or simultaneously received, a dividend on each outstanding share of Redeemable Preferred Stock in an amount at least equal to the greater of (i) $0.08 per share for Series A redeemable convertible preferred stock (“Series A preferred stock”), $0.104 per share for Series A-1 redeemable convertible preferred stock (“Series A-1 preferred stock”), $0.03251 per share for Series B redeemable convertible preferred stock (“Series B preferred stock”), $0.09437 per share for Series C-1 redeemable convertible preferred stock (“Series C-1 preferred stock”), $0.09437 per share for Series C-2 redeemable convertible preferred stock (“Series C-2 preferred stock”), $0.09437 per share for Series D redeemable convertible preferred stock (“Series D preferred stock”), Series E preferred stock and Series E-2 preferred stock, $0.40 per share for Series D-1 redeemable convertible preferred stock (“Series D-1 preferred stock”), and $0.1088 per share for Series E-1 preferred stock, Series E-3 preferred stock and Series F preferred stock and (ii) (A) in the case of a dividend on common stock or any class or series of stock that is convertible into common stock, that dividend per share of Redeemable Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Redeemable Preferred Stock, or (B) in the case of a dividend on any class or series that is not convertible into common stock, at a rate per share of Redeemable Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price (as defined below) of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Redeemable Preferred Stock. If the Company declared, paid or set aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Redeemable Preferred Stock would be calculated based upon the dividend on the class or series of capital stock that would result in the highest Redeemable Preferred Stock dividend. Stockholders were not entitled to any accruing dividends. No dividends have been declared or paid during the three or nine months ended September 30, 2017 or 2016.

The Original Issue Price per share was $1.00 for Series A, $1.30 for Series A-1, $0.4064 for Series B, $1.179633 for Series C-1, $1.179633 for Series C-2, $1.179633 for Series D, $5.50 for Series D-1, $1.179633 for Series E, $1.36 for Series E-1, $1.179633 for Series E-2, $1.36 for Series E-3 and $1.36 for Series F preferred stock, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization with respect to the Redeemable Preferred Stock.

Liquidation Preference

In the event of any liquidation event, voluntary or involuntary, dissolution or winding up of the Company or Deemed Liquidation Event (as defined below), the holders of the then outstanding Series F preferred stock were entitled to receive, prior and in preference to any distributions to the holders of the common stock and other preferred stock, $1.36 per share, plus any dividends declared but unpaid on the Series F preferred stock.

After the payment of all preferential amounts to the holders of Series F preferred stock, then, to the extent available, the holders of the Series E-2 and Series E-3 preferred stock would have been paid $1.179633 per share and $1.36 per share, respectively, plus any dividends declared but unpaid on the Series E-2 and Series E-3 preferred stock, prior and in preference to any distributions to the holders of common stock, Series E, Series E-1, Series D and Series D-1 preferred stock and Junior Preferred Stock.

After the payment of all preferential amounts to the holders of Series E-2 and Series E-3 preferred stock, then, to the extent available, the holders of the Series E preferred stock and the Series E-1 preferred stock would have been entitled to receive, prior and in preference to any distributions to the holders of the common stock and other preferred stock, $1.179633 per share and $1.36 per share, respectively, plus any dividends declared but unpaid on the Series E and Series E-1 preferred stock, prior and in preference to any distributions to the holders of common stock, Series D and Series D-1 preferred stock and Junior Preferred Stock.

After the payment of all preferential amounts to the holders of Series E and Series E-1 preferred stock, then, to the extent available, the holders of the Series D and Series D-1 preferred stock would have been paid $1.179633 per share and $5.50 per share, respectively, plus any dividends declared but unpaid on the Series D and Series D-1 preferred stock, prior and in preference to any distributions to the holders of common stock and Junior Preferred Stock.

After the payment of all preferential amounts to the holders of the Senior Preferred Stock, then, to the extent available, the holders of Series C-1 and Series C-2 preferred stock would have been paid $1.179633 per share plus any dividends declared but unpaid on the Series C-1 and Series C-2 preferred stock, prior and in preference to any distributions to the holders of common stock and Series B, Series A and Series A-1 preferred stock.

After the payment of all preferential amounts to the holders of the Senior Preferred Stock, Series C-1 and Series C-2 preferred stock, then, to the extent available, the holders of Series B preferred stock would have been paid $0.4064 per share plus any dividends declared but unpaid on the Series B preferred stock, prior and in preference to any distributions to the holders of common stock and Series A and Series A-1 preferred stock.

After the payment of all preferential amounts to the holders of the Senior Preferred Stock, Series C-1, Series C-2 and Series B preferred stock, then, to the extent available, the holders of Series A and Series A-1 preferred stock would have been paid $1.00 and $1.30 per share, respectively, plus any dividends declared but unpaid on the Series A and A-1 preferred stock, prior and in preference to any distributions to the holders of common stock.

After payments have been made in full to the holders of the Redeemable Preferred Stock, then, to the extent available, the remaining amounts would have been distributed among the holders of the shares of preferred stock and common stock, pro rata based on the number of shares held by each holder, treating for this purpose all such securities as if they had been converted to common stock immediately prior to such dissolution, liquidation or winding up of the Company.

Unless 55% of the holders of the Senior Preferred Stock, voting together as a single class, elected otherwise, a Deemed Liquidation Event would have included a merger or consolidation (other than one in which stockholders of the Company own a majority by voting power of the outstanding shares of the surviving or acquiring corporation) or a sale, lease, transfer, exclusive license or other disposition of substantially all of the assets of the Company.

Conversion

Each share of Redeemable Preferred Stock was convertible, at the option of the holder, at any time, and without the payment of additional consideration, or would have automatically converted into shares of common stock at the applicable conversion ratio then in effect (i) upon the closing of a firm commitment underwritten public offering at a price per share to the public, which when multiplied by the total number of shares of common stock then outstanding or then issuable upon conversion of outstanding Redeemable Preferred Stock immediately prior to the consummation of the offering, exceeded $150,000 and with at least $50,000 of gross proceeds to the Company or (ii) upon the vote or written consent of the holders of at least 55% of the outstanding shares of the Senior Preferred Stock, voting together as a single class. All shares that were required to be surrendered per the provisions above would have been deemed to have been retired and canceled and would not be reissued as shares of preferred stock.

The conversion ratio of each series of Redeemable Preferred Stock was determined by dividing the Original Issue Price of each series of preferred stock by the Conversion Price of each series, except for Series D-1 preferred stock. The conversion ratio for Series D-1 was determined by dividing $11.722013 by the Series D Conversion Price. The Conversion Price was $99.37 for Series A, $129.181 for Series A-1, $4.038397 for Series B, $11.722013 for Series C-1, Series C-2, Series D, Series E and Series E-2, and $13.51432 for Series E-1, Series E-3 and Series F. The Conversion Price was subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization and other adjustments as set forth in the Company’s certificate of incorporation, as amended and restated.

Redemption Rights

At the written election of at least 55% of the holders of the Senior Preferred Stock, voting together as a single class, the shares of Redeemable Preferred Stock outstanding were redeemable, at any time on or after December 22, 2020, in three equal annual installments commencing 60 days after receipt of the required vote, in an amount equal to the Original Issue Price per share of each series of Redeemable Preferred Stock plus all declared but unpaid dividends thereon.

Stock-Based Awards

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Awards

9. Stock-Based Awards

2017 Stock Incentive Plan

The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) was approved by the Company’s stockholders on June 16, 2017 and became effective on June 28, 2017. Under the 2017 Plan, the Company may grant incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, awards of restricted stock units and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2017 Plan; however, incentive stock options may only be granted to employees. The 2017 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The number of shares of common stock covered by options and the date those options become exercisable, type of options to be granted, exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2017 Plan with service-based vesting conditions generally vest over four years and may not have a duration in excess of ten years, although options have been granted with vesting terms of less than four years.

The total number of shares of common stock that may be issued under the 2017 Plan was (1) 1,244,816; plus (2) the number of shares equal to the sum of the number of shares of common stock then available for issuance under the 2016 plan, which was 424,601 shares, and the number of shares of common stock subject to outstanding awards under the 2006 plan and the 2016 plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right. The number of shares of common stock that may be issued under the 2017 Plan will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 1,244,816 shares, (ii) 4% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors.

During the three months ended September 30, 2017, pursuant to the terms of the 2017 Plan, the Company granted options to employees and directors to purchase 619,027 shares of common stock at a weighted average exercise price of $12.80 per share.

Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards.

The exercise price for stock options granted may not be less than the fair market value of the common stock as of the date of grant.

2017 Employee Stock Purchase Plan

On June 16, 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on June 28, 2017. A total of 150,000 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2017 ESPP will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 622,408 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors.

2016 Stock Incentive Plan

The Company’s 2016 Stock Incentive Plan (the “2016 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2016 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2016 Plan with service-based vesting conditions vest over four years and expire after ten years.

As of the effective date of the 2017 Plan, no stock options or other awards were made under the 2016 Plan. No shares remained available for future issuance as of September 30, 2017.

Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards under the 2017 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2017 Plan.

2006 Stock Incentive Plan

The Company’s 2006 Stock Incentive Plan, as amended, (the “2006 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2006 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2006 Plan with service-based vesting conditions generally vest over four years and expire after ten years, although options have been granted with vesting terms of less than four years.

The 2006 Plan expired in 2016. No shares remained available for future issuance as of December 31, 2016.

Stock Option Valuation

The assumptions that the Company used to determine the grant-date fair value of the stock options granted to employees and directors during the nine months ended September 30, 2017 and 2016 were as follows, presented on a weighted average basis:

	Nine Months Ended September 30, 2017			Nine Months Ended September 30, 2016
Risk-free interest rate		2.15	%		1.37	%
Expected term (in years)		6.1			6.1
Expected volatility		80.6	%		80.5	%
Expected dividend yield		0	%		0	%

Stock Options

The following table summarizes the Company’s stock option activity since January 1, 2017:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding at December 31, 2016		920,731		$	4.36		6.4	$	1,297
Granted		1,207,728			10.50
Exercised		(26,600	)		3.76
Forfeited		(46,595	)		5.14
Outstanding at September 30, 2017		2,055,264		$	7.96		8.2	$	11,385

Options exercisable at September 30, 2017		711,428		$	4.33		5.9	$	6,437
Options vested and expected to vest at September 30, 2017		1,998,844		$	7.87		8.1	$	11,239
Options exercisable at December 31, 2016		585,959		$	3.78		5.2	$	1,136
Options vested and expected to vest at December 31, 2016		904,010		$	4.37		6.3	$	1,289

The weighted average grant-date fair value of stock options granted during the nine months ended September 30, 2017 and 2016 was $7.51 and $3.24, respectively.

The aggregate fair value of stock options that vested during the nine months ended September 30, 2017 and 2016 was $810 and $718, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2017 and 2016 was $256 and $6, respectively.

Stock-Based Compensation

The Company recorded stock-based compensation expense related to stock options in the following expense categories of its statements of operations and comprehensive loss:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development expenses	$	191	$	54	$	396	$	158
General and administrative expenses		425		135		688		361
	$	616	$	189	$	1,084	$	519

As of September 30, 2017, the Company had an aggregate of $9,114 of unrecognized stock-based compensation expense, which it expects to recognize over a weighted average period of 3.5 years.

Net Loss per Share

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Net Loss per Share

10. Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Numerator:
Net loss	$	(6,259	)	$	(4,222	)	$	(15,739	)	$	(13,681	)
Accretion of redeemable convertible preferred stock to redemption value		—			(19	)		(41	)		(57	)
Net loss attributable to common stockholders	$	(6,259	)	$	(4,241	)	$	(15,780	)	$	(13,738	)

Denominator:
Weighted average common shares outstanding— basic and diluted		13,939,950			430,929			4,990,535			428,770

Net loss per share attributable to common stockholders— basic and diluted	$	(0.45	)	$	(9.84	)	$	(3.16	)	$	(32.04	)

The Company’s potential dilutive securities, which include stock options and redeemable convertible preferred stock, have been excluded from the computation of diluted net loss per share attributable to common stockholders whenever the effect of including them would be to reduce the net loss per share. In periods where there is a net loss, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following potential shares of common stock, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Three Months and Nine Months Ended September 30,
	2017		2016
Stock options to purchase common stock		2,055,264		981,951
Redeemable convertible preferred stock (as converted to common stock)		—		9,057,339
		2,055,264		10,039,290

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Operating Leases In February 2010, the Company entered into an operating lease agreement for office and laboratory space, which, as amended, expires in May 2018. Upon entering into the agreement, the Company was required to maintain a security deposit of $776, which was recorded as restricted cash in the Company’s balance sheet. In connection with an amendment to the operating lease agreement in January 2015, the required security deposit was reduced from $776 to $63, and $713 of restricted cash was released to the Company. In connection with an amendment to the operating lease agreement in June 2011, the landlord agreed to fund up to $752 in improvements to the leased facility, which was recorded as a liability and is being recognized as a reduction of rent expense over the remaining lease term. The agreement requires future minimum lease payments for the years ending December 31, 2017 (remaining three months) and 2018 of $135 and $202, respectively. The Company recognizes rent expense on a straight-line basis over the lease period and has recorded deferred rent for rent expense incurred but not yet paid. Rental expense under operating leases totaled $127 and $106 for the three months ended September 30, 2017 and 2016, respectively, and $367 and $316 for the nine months ended September 30, 2017 and 2016, respectively. Intellectual Property Licenses Harvard and Dana-Farber Agreement In August 2006, the Company entered into an exclusive license agreement with President and Fellows of Harvard College (“Harvard”) and Dana-Farber Cancer Institute (“DFCI”). The agreement granted the Company an exclusive worldwide license, with the right to sublicense, under specified patents and patent applications to develop, obtain regulatory approval for and commercialize specified product candidates based on stapled peptides. Under the agreement, the Company is obligated to use commercially reasonable efforts to develop and commercialize one or more licensed products and to achieve specified milestone events by specified dates. In connection with entering into the agreement, the Company paid an upfront license fee and issued to Harvard and DFCI shares of its common stock. In February 2010, the agreement was amended and restated (the “Harvard/DFCI agreement”) under which additional patent rights were added to the scope of the license agreement and the annual license maintenance fees were increased. Under the Harvard/DFCI agreement, the Company is obligated to make aggregate milestones payments of up to $7,700 per licensed therapeutic product upon the Company’s achievement of specified clinical, regulatory and sales milestones with respect to such product and up to $700 per licensed diagnostic product upon the Company’s achievement of specified regulatory and sales milestones with respect to such product. In addition, the Company is obligated to pay royalties of low single-digit percentages on annual net sales of licensed products sold by the Company, its affiliates or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis, and may be reduced in specified circumstances. In addition, the agreement obligates the Company to pay a percentage, up to the mid-twenties, of fees received by the Company in connection with its sublicense of the licensed products. In accordance with the terms of the agreement, the Company’s sublicense payment obligations may be subject to specified reductions. The Harvard/DFCI agreement requires the Company to pay annual license maintenance fees of $145 each year. Any payments made in connection with the annual license maintenance fees will be credited against any royalties due. The Company did not incur any license fees during the three months ended September 30, 2017 and 2016 and incurred license fees of $145 during each of the nine months ended September 30, 2017 and 2016. In addition, the Company did not make any milestone payments during the three and nine months ended September 30, 2017 and 2016. As of September 30, 2017, no additional milestones had been achieved and no liabilities for additional milestone payments had been recorded in the Company’s financial statements. Through September 30, 2017 and December 31, 2016, the Company had made non-refundable cash payments, consisting of license and maintenance fees, milestone payments and sublicense fees, totaling $4,428 and $4,283, respectively. As of September 30, 2017, the Company had not developed a commercial product using the licensed technologies and no royalties under the agreement had been paid or were due. Under the Harvard/DFCI agreement, the Company is responsible for all patent expenses related to the prosecution and maintenance of the licensed patents and applications in-licensed under the agreement as well as cost reimbursement of amounts incurred for all documented patent-related expenses. The agreement will expire on a product-by-product and country-by-country basis upon the last to expire of any valid patent claim pertaining to licensed products covered under the agreement. Materia Agreement In December 2006, the Company entered into a license agreement (the “Materia agreement”) with Materia, Inc. (“Materia”), under which it was granted a non-exclusive worldwide license, with the right to sublicense, under specified patent and patent applications to utilize Materia’s catalysts to develop, obtain regulatory approval for and commercialize specified peptides owned or controlled by Materia and the right to manufacture specified compositions owned or controlled by Materia. Under the Materia agreement, the Company is obligated to make aggregate milestone payments to Materia of up to $6,400 upon the Company’s achievement of specified clinical, regulatory and sales milestones with respect to each licensed product. In addition, the Company is obligated to pay tiered royalties ranging in the low single-digit percentages on annual net sales of licensed products sold by the Company or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis, and may be reduced in specified circumstances. The Materia agreement requires the Company to pay annual license fees of $50. The Company did not incur any license fees during the three and nine months ended September 30, 2017 and 2016, respectively. In addition, the Company did not make any milestone payments during the three and nine months ended September 30, 2017 and 2016. As of September 30, 2017, no additional milestones had been achieved and no liabilities for additional milestone payments had been recorded in the Company’s financial statements. The agreement expires upon the expiration of the Company’s obligation to pay royalties in each territory covered under the agreement. Scripps Agreement In October 2010, the Company entered into a patent license agreement (the “Scripps agreement”) with The Scripps Research Institute (“Scripps”) under which it was granted a license, with the right to sublicense, for the exclusive worldwide rights to utilize Scripps’ “Click” chemistry for therapeutics and non-exclusive worldwide rights for diagnostics with the Company’s stabilized peptide and protein technology platforms. Under the agreement, the Company is obligated to make aggregate milestone payments to Scripps of up to $1,900 for each licensed peptide product and up to $950 for each licensed protein product upon achieving of specified clinical, regulatory and commercial milestones. In addition, the Company is obligated to pay tiered royalties ranging in the low single-digit percentages on annual net sales of licensed products sold by the Company or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis. The Scripps agreement requires the Company to pay annual license fees of $50. The Company did not incur any license fees during the three and nine months ended September 30, 2017 and 2016, respectively. As of September 30, 2017, no milestones had been achieved and no liabilities for milestone payments had been recorded in the Company’s financial statements. As of September 30, 2017, the Company had not developed a commercial product using the licensed technologies and no royalties under the agreement had been paid or were due. The agreement expires upon expiration of the last of any patent rights covered under the agreement. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it had not accrued any liabilities related to such obligations in its financial statements as of September 30, 2017 or December 31, 2016.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2017

Accounting Policies [Abstract]

Use of Estimates

Unaudited Interim Financial Information

Cash Equivalents

Restricted Cash

Investments

The Company classifies its available-for-sale investments as current assets on the balance sheet if they mature within one year from the balance sheet date.

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Deferred Offering Costs

Net Income (Loss) per Share

Recently Issued Accounting Pronouncements

Fair Value of Financial Assets (Tables)

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Summary of Assets Measured at Fair Value on Recurring Basis

	Fair Value Measurements as of September 30, 2017 using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	10,321	$	—	$	—	$	10,321
Corporate notes		—		1,000		—		1,000
Commercial paper		—		1,998		—		1,998
Investments:
Corporate notes		—		24,910		—		24,910
Commercial paper		—		18,527		—		18,527
	$	10,321	$	46,435	$	—	$	56,756

	Fair Value Measurements as of December 31, 2016 using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	20,016	$	—	$	—	$	20,016
	$	20,016	$	—	$	—	$	20,016

Investments (Tables)

9 Months Ended

Sep. 30, 2017

Investments Debt And Equity Securities [Abstract]

Summary of Fair Value of Available for Sale Investments

As of December 31, 2016, the Company had no available-for-sale investments. As of September 30, 2017, the fair value of available-for-sale investments by type of security was as follows:

	September 30, 2017
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Corporate notes	$	18,530	$	1	$	(4	)	$	18,527
Commercial paper		24,913		1		(4	)		24,910
	$	43,443	$	2	$	(8	)	$	43,437

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Redeemable Convertible Preferred Stock (Tables)

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Schedule of Redeemable Preferred Stock

Redeemable Preferred Stock consisted of the following as of:

	December 31, 2016
	Preferred Shares Authorized		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		1,250,000		1,250,000	$	1,250	$	1,250		12,579
Series A-1 preferred stock		615,384		615,384		800		800		6,192
Series B preferred stock		3,706,056		3,706,056		1,506		1,506		372,955
Series C-1 preferred stock		5,934,050		5,934,050		6,997		7,000		597,167
Series C-2 preferred stock		8,689,144		8,689,144		10,248		10,250		874,423
Series D preferred stock		34,142,865		34,142,865		40,263		40,276		3,435,932
Series D-1 preferred stock		363,636		363,636		2,000		2,000		36,594
Series E preferred stock		12,715,822		3,788,240		4,453		4,469		381,225
Series E-1 preferred stock		24,264,705		7,697,597		10,446		10,469		774,639
Series E-2 preferred stock		9,226,082		8,927,582		10,493		10,531		898,418
Series E-3 preferred stock		21,237,785		16,567,108		22,483		22,531		1,667,214
Series F preferred stock		29,411,764		13,949,357		18,806		18,971		1,403,779
		151,557,293		105,631,019	$	129,745	$	130,053		10,461,117

Stock-Based Awards (Tables)

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Valuation Assumptions of Stock Options Granted

	Nine Months Ended September 30, 2017			Nine Months Ended September 30, 2016
Risk-free interest rate		2.15	%		1.37	%
Expected term (in years)		6.1			6.1
Expected volatility		80.6	%		80.5	%
Expected dividend yield		0	%		0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity since January 1, 2017:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding at December 31, 2016		920,731		$	4.36		6.4	$	1,297
Granted		1,207,728			10.50
Exercised		(26,600	)		3.76
Forfeited		(46,595	)		5.14
Outstanding at September 30, 2017		2,055,264		$	7.96		8.2	$	11,385

Options exercisable at September 30, 2017		711,428		$	4.33		5.9	$	6,437
Options vested and expected to vest at September 30, 2017		1,998,844		$	7.87		8.1	$	11,239
Options exercisable at December 31, 2016		585,959		$	3.78		5.2	$	1,136
Options vested and expected to vest at December 31, 2016		904,010		$	4.37		6.3	$	1,289

Summary of Stock Based Compensation Expense

The Company recorded stock-based compensation expense related to stock options in the following expense categories of its statements of operations and comprehensive loss:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development expenses	$	191	$	54	$	396	$	158
General and administrative expenses		425		135		688		361
	$	616	$	189	$	1,084	$	519

Net Loss per Share (Tables)

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Basic and Diluted Net Loss per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Numerator:
Net loss	$	(6,259	)	$	(4,222	)	$	(15,739	)	$	(13,681	)
Accretion of redeemable convertible preferred stock to redemption value		—			(19	)		(41	)		(57	)
Net loss attributable to common stockholders	$	(6,259	)	$	(4,241	)	$	(15,780	)	$	(13,738	)

Denominator:
Weighted average common shares outstanding— basic and diluted		13,939,950			430,929			4,990,535			428,770

Net loss per share attributable to common stockholders— basic and diluted	$	(0.45	)	$	(9.84	)	$	(3.16	)	$	(32.04	)

Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share

The following potential shares of common stock, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Three Months and Nine Months Ended September 30,
	2017		2016
Stock options to purchase common stock		2,055,264		981,951
Redeemable convertible preferred stock (as converted to common stock)		—		9,057,339
		2,055,264		10,039,290

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands			9 Months Ended
	Jul. 05, 2017 USD ($) $ / shares shares	Jun. 16, 2017	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) shares
Class of Stock [Line Items]
Offering expenses			$ 2,049	$ 180
Redeemable convertible preferred stock, outstanding (in shares) \| shares			0		105,631,019
Number of common shares issued on conversion of redeemable convertible preferred stock \| shares					10,461,117
Proceeds from collaboration agreement			$ 34,900
Accumulated deficit			(130,081)		$ (114,342)
Cash, cash equivalents and investments			56,865
Preferred Stock [Member]
Class of Stock [Line Items]
Proceeds from sales of stock			$ 131,200
IPO [Member] \| Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Redeemable convertible preferred stock, outstanding (in shares) \| shares	106,114,520
Common Stock [Member]
Class of Stock [Line Items]
Stock issued and sold \| shares			3,750,000
Common Stock [Member] \| IPO [Member]
Class of Stock [Line Items]
Reverse stock split description			one-for-9.937 reverse stock split
Reverse stock split ratio		0.100634
Stock issued and sold \| shares	3,750,000
Share price \| $ / shares	$ 15.00
Net proceeds from issuance of initial public offering	$ 50,000
Underwriting discounts and commissions incurred	3,900
Offering expenses	$ 2,300
Number of common shares issued on conversion of redeemable convertible preferred stock \| shares	10,509,774

Redeemable Convertible Preferred Stock - Additional Information (Detail)		1 Months Ended			3 Months Ended		9 Months Ended
	Jul. 05, 2017 USD ($) $ / shares shares	Feb. 28, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Jan. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) Installment $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Director Installment $ / shares shares	Sep. 30, 2016 USD ($)
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			151,557,293		0		0
Redeemable convertible preferred stock, par value (in dollars per share) \| $ / shares			$ 0.01		$ 0.01		$ 0.01
Preferred stock shares issued							483,501
Proceeds from issuance of redeemable convertible preferred stock \| $							$ 471,000	$ 13,183,000
Preferred stock issuance costs, paid \| $							$ 2,049,000	180,000
Number of common shares issued on conversion of redeemable convertible preferred stock			10,461,117
Redeemable convertible preferred stock, issued (in shares)			105,631,019		0		0
Redeemable convertible preferred stock, outstanding (in shares)			105,631,019		0		0
Preferred stock, voting rights description							The holders of the Redeemable Preferred Stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote and had the right to vote the number of shares equal to the number of whole shares of common stock into which such holders of Redeemable Preferred Stock could convert on the record date for determination of stockholders entitled to vote. In addition, holders of the Senior Preferred Stock, voting as a single class, were entitled to elect three directors of the Company. The holders of the Junior Preferred Stock, voting as a single class, were entitled to elect two directors of the Company.
Dividends declared \| $					$ 0	$ 0	$ 0	0
Dividends paid \| $					$ 0	$ 0	$ 0	$ 0
Percentage of voting to be held by holders of senior preferred stock for deemed liquidation event							55.00%
Redemption date of redeemable preferred stock							Dec. 22, 2020
Number of annual installments for redemption \| Installment					3		3
IPO [Member]
Temporary Equity [Line Items]
Initial public offering completion date							Jul. 05, 2017
IPO [Member] \| Common Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued, price per share \| $ / shares	$ 15.00
Preferred stock issuance costs, paid \| $	$ 2,300,000
Number of common shares issued on conversion of redeemable convertible preferred stock	10,509,774
Underwritten public offering costs \| $	$ 3,900,000
Series E-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued				9,705,882
Preferred stock shares issued, price per share \| $ / shares				$ 1.36
Proceeds from issuance of redeemable convertible preferred stock \| $				$ 13,183,000
Preferred stock issuance costs, paid \| $				$ 17,000
Preferred stock shares converted		4,411,765	16,567,108
Series F Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued		483,501	13,949,357
Preferred stock shares issued, price per share \| $ / shares		$ 1.36	$ 1.36
Proceeds from issuance of redeemable convertible preferred stock \| $		$ 626,000	$ 18,806,000
Preferred stock issuance costs, paid \| $		$ 32,000	$ 165,000
Stock purchase agreement, additional shares agreed to purchase		274,957	8,192,477
Stock purchase agreement, price per share \| $ / shares		$ 1.36	$ 1.36
Stock purchase agreement, aggregate purchase price \| $		$ 11,516,000	$ 11,142,000
Stock purchase agreement, shares authorized		758,458
Stock purchase agreement, aggregate number of shares agreed to purchase		8,467,434
Series E- Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares converted			8,927,582
Series E-2 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued on conversion			8,927,582
Series E-3 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued on conversion		4,411,765	16,567,108
Senior Preferred Stock [Member]
Temporary Equity [Line Items]
Number of directors entitled to be elected \| Director							3
Junior Preferred Stock [Member]
Temporary Equity [Line Items]
Number of directors entitled to be elected \| Director							2
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			1,250,000
Number of common shares issued on conversion of redeemable convertible preferred stock			12,579
Redeemable convertible preferred stock, issued (in shares)			1,250,000
Redeemable convertible preferred stock, outstanding (in shares)			1,250,000
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							$ 0.08
Original issue price per share \| $ / shares							1.00
Liquidation preference per share \| $ / shares					$ 1.00		1.00
Conversion price of Redeemable Preferred \| $ / shares					99.37		99.37
Series A-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			615,384
Number of common shares issued on conversion of redeemable convertible preferred stock			6,192
Redeemable convertible preferred stock, issued (in shares)			615,384
Redeemable convertible preferred stock, outstanding (in shares)			615,384
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.104
Original issue price per share \| $ / shares							1.30
Liquidation preference per share \| $ / shares					1.30		1.30
Conversion price of Redeemable Preferred \| $ / shares					129.181		129.181
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			3,706,056
Number of common shares issued on conversion of redeemable convertible preferred stock			372,955
Redeemable convertible preferred stock, issued (in shares)			3,706,056
Redeemable convertible preferred stock, outstanding (in shares)			3,706,056
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.03251
Original issue price per share \| $ / shares							0.4064
Liquidation preference per share \| $ / shares					0.4064		0.4064
Conversion price of Redeemable Preferred \| $ / shares					4.038397		4.038397
Series C-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			5,934,050
Number of common shares issued on conversion of redeemable convertible preferred stock			597,167
Redeemable convertible preferred stock, issued (in shares)			5,934,050
Redeemable convertible preferred stock, outstanding (in shares)			5,934,050
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.09437
Original issue price per share \| $ / shares							1.179633
Liquidation preference per share \| $ / shares					1.179633		1.179633
Conversion price of Redeemable Preferred \| $ / shares					11.722013		11.722013
Series C-2 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			8,689,144
Number of common shares issued on conversion of redeemable convertible preferred stock			874,423
Redeemable convertible preferred stock, issued (in shares)			8,689,144
Redeemable convertible preferred stock, outstanding (in shares)			8,689,144
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.09437
Original issue price per share \| $ / shares							1.179633
Liquidation preference per share \| $ / shares					1.179633		1.179633
Conversion price of Redeemable Preferred \| $ / shares					11.722013		11.722013
Series D Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			34,142,865
Number of common shares issued on conversion of redeemable convertible preferred stock			3,435,932
Redeemable convertible preferred stock, issued (in shares)			34,142,865
Redeemable convertible preferred stock, outstanding (in shares)			34,142,865
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.09437
Original issue price per share \| $ / shares							1.179633
Liquidation preference per share \| $ / shares					1.179633		1.179633
Conversion price of Redeemable Preferred \| $ / shares					11.722013		11.722013
Series D-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			363,636
Number of common shares issued on conversion of redeemable convertible preferred stock			36,594
Redeemable convertible preferred stock, issued (in shares)			363,636
Redeemable convertible preferred stock, outstanding (in shares)			363,636
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.40
Original issue price per share \| $ / shares							5.50
Liquidation preference per share \| $ / shares					5.50		5.50
Series E-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			24,264,705
Number of common shares issued on conversion of redeemable convertible preferred stock			774,639
Redeemable convertible preferred stock, issued (in shares)			7,697,597
Redeemable convertible preferred stock, outstanding (in shares)			7,697,597
Noncumulative dividends entitled to receive by redeemable preferred stock holders \| $ / shares							0.1088
Original issue price per share \| $ / shares							1.36
Liquidation preference per share \| $ / shares					1.36		1.36
Conversion price of Redeemable Preferred \| $ / shares					13.51432		13.51432
Series E Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			12,715,822
Number of common shares issued on conversion of redeemable convertible preferred stock			381,225
Redeemable convertible preferred stock, issued (in shares)			3,788,240
Redeemable convertible preferred stock, outstanding (in shares)			3,788,240
Original issue price per share \| $ / shares							1.179633
Liquidation preference per share \| $ / shares					1.179633		1.179633
Conversion price of Redeemable Preferred \| $ / shares					11.722013		11.722013
Series E-2 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			9,226,082
Number of common shares issued on conversion of redeemable convertible preferred stock			898,418
Redeemable convertible preferred stock, issued (in shares)			8,927,582
Redeemable convertible preferred stock, outstanding (in shares)			8,927,582
Original issue price per share \| $ / shares							1.179633
Liquidation preference per share \| $ / shares					1.179633		1.179633
Conversion price of Redeemable Preferred \| $ / shares					11.722013		11.722013
Series E-3 Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			21,237,785
Number of common shares issued on conversion of redeemable convertible preferred stock			1,667,214
Redeemable convertible preferred stock, issued (in shares)			16,567,108
Redeemable convertible preferred stock, outstanding (in shares)			16,567,108
Original issue price per share \| $ / shares							1.36
Liquidation preference per share \| $ / shares					1.36		1.36
Conversion price of Redeemable Preferred \| $ / shares					13.51432		13.51432
Series F Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)			29,411,764
Number of common shares issued on conversion of redeemable convertible preferred stock			1,403,779
Redeemable convertible preferred stock, issued (in shares)			13,949,357
Redeemable convertible preferred stock, outstanding (in shares)			13,949,357
Original issue price per share \| $ / shares							1.36
Liquidation preference per share \| $ / shares					1.36		1.36
Conversion price of Redeemable Preferred \| $ / shares					$ 13.51432		$ 13.51432
Redeemable Convertible Preferred Stock [Member] \| Maximum [Member]
Temporary Equity [Line Items]
Underwritten public offering costs \| $							$ 150,000,000
Redeemable Convertible Preferred Stock [Member] \| Minimum [Member]
Temporary Equity [Line Items]
Gross proceeds from issuance of redeemable convertible preferred stock \| $							$ 50,000,000
Redeemable Convertible Preferred Stock [Member] \| IPO [Member]
Temporary Equity [Line Items]
Redeemable convertible preferred stock, outstanding (in shares)	106,114,520

Redeemable Convertible Preferred Stock - Schedule of Redeemable Preferred Stock (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Preferred Shares Authorized	0	151,557,293
Preferred Shares Issued	0	105,631,019
Preferred Shares Outstanding	0	105,631,019
Carrying Value		$ 129,745
Liquidation Preference		$ 130,053
Common Stock Issuable Upon Conversion		10,461,117
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		1,250,000
Preferred Shares Issued		1,250,000
Preferred Shares Outstanding		1,250,000
Carrying Value		$ 1,250
Liquidation Preference		$ 1,250
Common Stock Issuable Upon Conversion		12,579
Series A-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		615,384
Preferred Shares Issued		615,384
Preferred Shares Outstanding		615,384
Carrying Value		$ 800
Liquidation Preference		$ 800
Common Stock Issuable Upon Conversion		6,192
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		3,706,056
Preferred Shares Issued		3,706,056
Preferred Shares Outstanding		3,706,056
Carrying Value		$ 1,506
Liquidation Preference		$ 1,506
Common Stock Issuable Upon Conversion		372,955
Series C-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		5,934,050
Preferred Shares Issued		5,934,050
Preferred Shares Outstanding		5,934,050
Carrying Value		$ 6,997
Liquidation Preference		$ 7,000
Common Stock Issuable Upon Conversion		597,167
Series C-2 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		8,689,144
Preferred Shares Issued		8,689,144
Preferred Shares Outstanding		8,689,144
Carrying Value		$ 10,248
Liquidation Preference		$ 10,250
Common Stock Issuable Upon Conversion		874,423
Series D Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		34,142,865
Preferred Shares Issued		34,142,865
Preferred Shares Outstanding		34,142,865
Carrying Value		$ 40,263
Liquidation Preference		$ 40,276
Common Stock Issuable Upon Conversion		3,435,932
Series D-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		363,636
Preferred Shares Issued		363,636
Preferred Shares Outstanding		363,636
Carrying Value		$ 2,000
Liquidation Preference		$ 2,000
Common Stock Issuable Upon Conversion		36,594
Series E Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		12,715,822
Preferred Shares Issued		3,788,240
Preferred Shares Outstanding		3,788,240
Carrying Value		$ 4,453
Liquidation Preference		$ 4,469
Common Stock Issuable Upon Conversion		381,225
Series E-1 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		24,264,705
Preferred Shares Issued		7,697,597
Preferred Shares Outstanding		7,697,597
Carrying Value		$ 10,446
Liquidation Preference		$ 10,469
Common Stock Issuable Upon Conversion		774,639
Series E-2 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		9,226,082
Preferred Shares Issued		8,927,582
Preferred Shares Outstanding		8,927,582
Carrying Value		$ 10,493
Liquidation Preference		$ 10,531
Common Stock Issuable Upon Conversion		898,418
Series E-3 Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		21,237,785
Preferred Shares Issued		16,567,108
Preferred Shares Outstanding		16,567,108
Carrying Value		$ 22,483
Liquidation Preference		$ 22,531
Common Stock Issuable Upon Conversion		1,667,214
Series F Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Shares Authorized		29,411,764
Preferred Shares Issued		13,949,357
Preferred Shares Outstanding		13,949,357
Carrying Value		$ 18,806
Liquidation Preference		$ 18,971
Common Stock Issuable Upon Conversion		1,403,779

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Jun. 16, 2017	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized for issuance		3,113,310	3,113,310		12,206,216
Common stock, weighted average exercise price per share			$ 10.50
Weighted average grant-date fair value of stock options			$ 7.51	$ 3.24
Aggregate fair value of stock options vested			$ 810	$ 718
Aggregate intrinsic value of stock options exercised			256	$ 6
Aggregate unrecognized stock-based compensation expense		$ 9,114	$ 9,114
Unrecognized stock-based compensation expense, weighted average period expects for recognition			3 years 6 months
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase			3,750,000
2017 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period			4 years
Number of shares, available for grant		1,244,816	1,244,816
Maximum annual increase in common stock reserved for future issuance			1,244,816
Percentage of common stock shares outstanding			4.00%
Options to purchase		619,027
Common stock, weighted average exercise price per share		$ 12.80
2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period			4 years
Stock option expiration period			10 years
Common stock, shares authorized for issuance		0	0
2016 Stock Incentive Plan [Member] \| Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized for issuance		424,601	424,601
2017 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized for issuance	150,000
Maximum annual increase in common stock reserved for future issuance	622,408
Percentage of common stock shares outstanding	1.00%
Stock incentive plan description			On June 16, 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on June 28, 2017. A total of 150,000 shares of common stock were initially reserved for issuance under this plan. The number of shares of common stock that may be issued under the 2017 ESPP will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 622,408 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors.
2006 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period					4 years
Stock option expiration period					10 years
Stock incentive plan termination year					2016
Maximum [Member] \| 2017 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period			4 years
Stock option expiration period			10 years
Maximum [Member] \| 2006 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period					4 years

Stock-Based Awards - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance \| shares	920,731
Number of Shares, Granted \| shares	1,207,728
Number of Shares, Exercised \| shares	(26,600)
Number of Shares, Forfeited \| shares	(46,595)
Number of Shares, Ending balance \| shares	2,055,264	920,731
Number of Shares, Options exercisable \| shares	711,428	585,959
Number of Shares, Options vested and expected to vest \| shares	1,998,844	904,010
Weighted Average Exercise Price, Beginning balance \| $ / shares	$ 4.36
Weighted Average Exercise Price, Granted \| $ / shares	10.50
Weighted Average Exercise Price, Exercised \| $ / shares	3.76
Weighted Average Exercise Price, Forfeited \| $ / shares	5.14
Weighted Average Exercise Price, Ending balance \| $ / shares	7.96	$ 4.36
Weighted Average Exercise Price, Options exercisable \| $ / shares	4.33	3.78
Weighted Average Exercise Price, Options vested and expected to vest \| $ / shares	$ 7.87	$ 4.37
Weighted-Average Remaining Contractual Term, Outstanding	8 years 2 months 12 days	6 years 4 months 24 days
Weighted Average Remaining Contractual Term, Options exercisable	5 years 10 months 24 days	5 years 2 months 12 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	8 years 1 month 7 days	6 years 3 months 19 days
Aggregate Intrinsic Value \| $	$ 11,385	$ 1,297
Aggregate Intrinsic Value, Options exercisable \| $	6,437	1,136
Aggregate Intrinsic Value, Options vested and expected to vest \| $	$ 11,239	$ 1,289

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Jan. 31, 2015 USD ($)	Feb. 28, 2010 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Milestone	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2011 USD ($)	Oct. 31, 2010 USD ($)	Dec. 31, 2006 USD ($)
Other Commitments [Line Items]
Restricted cash								$ 63,000
Rental expense under operating leases				$ 127,000	$ 106,000	$ 367,000	$ 316,000
Operating Lease Agreement [Member]
Other Commitments [Line Items]
Operating lease expiration	2018-05
Restricted cash		$ 63,000	$ 776,000
Restricted cash released		$ 713,000
Improvements to leased facility									$ 752,000
Future minimum lease payments, 2017	$ 135,000			135,000		135,000
Future minimum lease payments, 2018	202,000			202,000		202,000
Harvard and Dana-Farber Agreement [Member]
Other Commitments [Line Items]
Annual license maintenance fees			145,000
Milestone payments				0	0	0	0
License fees				0	0	$ 145,000	145,000
Milestones achieved \| Milestone						0
Additional liabilities for milestone payments	0			0		$ 0
Non-refundable cash payment						4,428,000		$ 4,283,000
Payments for royalties						0
Harvard and Dana-Farber Agreement [Member] \| Therapeutic Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments			7,700,000
Harvard and Dana-Farber Agreement [Member] \| Diagnostic Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments			$ 700,000
Materia Agreement [Member]
Other Commitments [Line Items]
Aggregate milestones payments											$ 6,400,000
Milestone payments				0	0	0	0
License fees				0	0	$ 0	0
Milestones achieved \| Milestone						0
Additional liabilities for milestone payments	0			0		$ 0
Annual license fees											$ 50,000
Scripps Agreement [Member]
Other Commitments [Line Items]
License fees				0	$ 0	$ 0	$ 0
Milestones achieved \| Milestone						0
Additional liabilities for milestone payments	$ 0			$ 0		$ 0
Payments for royalties						$ 0
Annual license fees										$ 50,000
Scripps Agreement [Member] \| Peptide Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments										1,900,000
Scripps Agreement [Member] \| Protein Product [Member]
Other Commitments [Line Items]
Aggregate milestones payments										$ 950,000

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes The Company did not provide for any income taxes for the three and nine months ended September 30, 2017 and 2016. The Company has evaluated the positive and negative evidence bearing upon the realizability of its U.S. net deferred tax assets. As required by the provisions of ASC 740, Income Taxes, management has determined that it is more-likely-than-not that the Company will not utilize the benefits of federal and state U.S. net deferred tax assets for financial reporting purposes. Accordingly, the net deferred tax assets are subject to a valuation allowance at September 30, 2017 and December 31, 2016.

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jul. 05, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		$ 88		$ 88			$ 25
Restricted cash, non-current							63
Deferred offering costs		0		0			0
General and administrative expense		2,601	$ 1,627	6,039	$ 6,537
2016 Equity Financing [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs								$ 1,500
IPO [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs						$ 2,304
Write-off of Deferred Financing Costs [Member]
Summary Of Significant Accounting Policies [Line Items]
General and administrative expense	$ 1,500
Security Deposit Related to Credit Card Accounts [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		25		25			25
Security Deposit Related to Lease Facilities [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash, current		$ 63		$ 63
Restricted cash, non-current							$ 63

Investments - Summary of Fair Value of Available for Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	$ 43,443
Investments, Gross Unrealized Gain	2
Investments, Gross Unrealized Loss	(8)
Investments, Fair Value	43,437	$ 0
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	18,530
Investments, Gross Unrealized Gain	1
Investments, Gross Unrealized Loss	(4)
Investments, Fair Value	18,527
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Amortized Cost	24,913
Investments, Gross Unrealized Gain	1
Investments, Gross Unrealized Loss	(4)
Investments, Fair Value	$ 24,910

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Payroll and payroll-related costs	$ 795	$ 899
External research and development services	1,602	723
Professional fees	472	322
Other	504	156
Total accrued expenses and other current liabilities	$ 3,373	$ 2,100

Stock-Based Awards - Valuation Assumptions of Stock Options Granted (Detail)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rate	2.15%	1.37%
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days
Expected volatility	80.60%	80.50%
Expected dividend yield	0.00%	0.00%

Condensed Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Statement Of Stockholders Equity [Abstract]
Issuance of Series F redeemable convertible preferred stock, Issuance costs	$ 32

Preferred Stock	9 Months Ended
Preferred Stock	Sep. 30, 2017
Equity [Abstract]
Preferred Stock	7. Preferred Stock On July 5, 2017, in connection with the closing of the Company’s IPO, the Company effected its amended and restated certificate of incorporation, which authorizes the Company to issue 5,000,000 shares of preferred stock, $0.001 par value per share.

Common Stock	9 Months Ended
Common Stock	Sep. 30, 2017
Equity [Abstract]
Common Stock	8. Common Stock As of December 31, 2016, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 143,500,000 shares of $0.001 par value common stock. On July 5, 2017, the Company increased the authorized number of shares of common stock to 150,000,000 shares. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the Company’s board of directors, if any, subject to the preferential dividend rights of the preferred stock. As of September 30, 2017 and December 31, 2016, no dividends had been declared. As of September 30, 2017, the Company had reserved 3,113,310 shares for the exercise of outstanding stock options and grant of future awards under the Company’s stock incentive plans (see Note 9). As of December 31, 2016, the Company had reserved 12,206,216 shares for the conversion of outstanding shares of Redeemable Preferred Stock (see Note 6), the exercise of outstanding stock options and the number of shares remaining available for future grant under the Company’s stock incentive plans (see Note 9). Upon completion of the Company’s IPO on July 5, 2017, all shares of Redeemable Preferred Stock converted to common stock.

Fair Value of Financial Assets - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	$ 56,756	$ 20,016
Corporate Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	24,910
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	18,527
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	10,321	20,016
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total assets	46,435
Level 2 [Member] \| Corporate Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	24,910
Level 2 [Member] \| Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	18,527
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	10,321	20,016
Money Market Funds [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	10,321	$ 20,016
Corporate Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,000
Corporate Notes [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,000
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	1,998
Commercial Paper [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	$ 1,998

Investments - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Investments, Fair Value	$ 43,437	$ 0