Global Blood Therapeutics (GBT) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 22, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	GBT
Entity Registrant Name	Global Blood Therapeutics, Inc.
Entity Central Index Key	1,629,137
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		47,063,836
Entity Public Float			$ 1,099,572,237

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 198,332	$ 92,072
Short-term marketable securities	116,493	55,202
Prepaid expenses	3,839	1,983
Other assets, current	5,648	512
Total current assets	324,312	149,769
Property and equipment, net	16,571	2,420
Long-term marketable securities	14,607	50,058
Restricted cash	1,046	140
Other assets, noncurrent	184
Total assets	356,720	202,387
Current liabilities:
Accounts payable	7,177	4,320
Accrued liabilities	10,135	5,319
Accrued compensation	8,579	4,967
Other liabilities, current	373	909
Total current liabilities	26,264	15,515
Other liabilities, noncurrent	11,652	563
Total liabilities	37,916	16,078
Commitments and contingencies (Note 11)
Stockholders' equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized at December 31, 2017 and 2016, respectively, and none issued and outstanding as of December 31, 2017 and 2016.
Common stock, $0.001 par value, 150,000,000 shares authorized at December 31, 2017 and 2016, respectively; 46,131,723 and 36,638,156 shares issued and outstanding at December 31, 2017 and 2016, respectively.	46	37
Additional paid-in capital	617,051	367,371
Accumulated other comprehensive loss	(336)	(166)
Accumulated deficit	(297,957)	(180,933)
Total stockholders' equity	318,804	186,309
Total liabilities and stockholders' equity	$ 356,720	$ 202,387

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
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	150,000,000
Common stock, shares issued	46,131,723	36,638,156
Common stock, shares outstanding	46,131,723	36,638,156

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 87,807	$ 62,163	$ 36,657
General and administrative	31,438	20,964	9,671
Related party expenses			65
Total operating expenses	119,245	83,127	46,393
Loss from operations	(119,245)	(83,127)	(46,393)
Interest income, net	2,555	659	33
Other expenses, net	(334)
Net loss	(117,024)	(82,468)	(46,360)
Other comprehensive loss:
Net unrealized loss on marketable securities, net of tax	(170)	(166)
Comprehensive loss	$ (117,194)	$ (82,634)	$ (46,360)
Basic and diluted net loss per common share	$ (2.76)	$ (2.48)	$ (3.95)
Weighted-average number of shares used in computing basic and diluted net loss per common share	42,323,686	33,207,382	12,806,697

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2014	$ 102,161
Beginning Balance, Shares at Dec. 31, 2014	69,113,168
Accretion of redeemable convertible preferred stock to redemption value	$ 4,180
Conversion of Series A and B redeemable convertible preferred stock into common stock	$ (106,341)
Conversion of Series A and B redeemable convertible preferred stock into common stock (shares)	(69,113,168)
Beginning Balance at Dec. 31, 2014	$ (49,326)	$ 2			$ (49,328)
Beginning Balance, Shares at Dec. 31, 2014		1,954,488
Accretion of redeemable convertible preferred stock to redemption value	(4,180)		$ (1,403)		(2,777)
Conversion of Series A and B redeemable convertible preferred stock into common stock	106,341	$ 20	106,321
Conversion of Series A and B redeemable convertible preferred stock into common stock, Shares		19,746,614
Issuance of common stock upon initial public offering and equity offerings, net of issuance costs	126,230	$ 7	126,223
Issuance of common stock upon initial public offering and equity offerings, net of issuance costs, Shares		6,900,000
Common stock issued for license	4,492		4,492
Common stock issued for license, Shares		85,714
Issuance of common stock upon exercise of stock options	45		45
Common stock issued on exercise of stock options, Shares		89,549
Vesting of restricted stock purchases	330		330
Vesting of restricted stock purchases, Shares		583,435
Stock-based compensation expense	3,223		3,223
Net loss	(46,360)				(46,360)
Ending Balance at Dec. 31, 2015	140,795	$ 29	239,231		(98,465)
Ending Balance, Shares at Dec. 31, 2015		29,359,800
Issuance of common stock upon initial public offering and equity offerings, net of issuance costs	116,995	$ 7	116,988
Issuance of common stock upon initial public offering and equity offerings, net of issuance costs, Shares		6,667,228
Issuance of common stock upon exercise of stock options	222		222
Common stock issued on exercise of stock options, Shares		147,126
Issuance of common stock pursuant to ESPP purchases	1,018		1,018
Issuance of common stock pursuant to ESPP purchases, Shares		65,252
Vesting of restricted stock purchases	678	$ 1	677
Vesting of restricted stock purchases, Shares		398,750
Stock-based compensation expense	9,235		9,235
Net unrealized gain (loss) on marketable securities	(166)			$ (166)
Net loss	(82,468)				(82,468)
Ending Balance at Dec. 31, 2016	$ 186,309	$ 37	367,371	(166)	(180,933)
Ending Balance, Shares at Dec. 31, 2016		36,638,156
Ending Balance, Shares at Dec. 31, 2017	0
Issuance of common stock upon initial public offering and equity offerings, net of issuance costs	$ 231,955	$ 8	231,947
Issuance of common stock upon initial public offering and equity offerings, net of issuance costs, Shares		8,498,926
Issuance of common stock upon exercise of stock options	$ 2,816	$ 1	2,815
Common stock issued on exercise of stock options, Shares	578,455	578,455
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	$ (238)		(238)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		33,212
Issuance of common stock pursuant to ESPP purchases	1,050		1,050
Issuance of common stock pursuant to ESPP purchases, Shares		76,585
Vesting of restricted stock purchases	424		424
Vesting of restricted stock purchases, Shares		306,389
Stock-based compensation expense	13,682		13,682
Net unrealized gain (loss) on marketable securities	(170)			(170)
Net loss	(117,024)				(117,024)
Ending Balance at Dec. 31, 2017	$ 318,804	$ 46	$ 617,051	$ (336)	$ (297,957)
Ending Balance, Shares at Dec. 31, 2017		46,131,723

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (117,024)	$ (82,468)	$ (46,360)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,658	1,160	873
Amortization of premium on marketable securities	704	75
Loss on disposal of fixed assets			33
Stock-based compensation	13,682	9,235	3,223
Fair value of stock issued for license			4,492
Changes in operating assets and liabilities:
Prepaid expenses	(1,856)	(761)	(87)
Other assets, current	(162)	(512)	22
Accounts payable	2,771	1,066	2,705
Payable due to related party			(14)
Accrued liabilities	3,280	1,794	2,834
Accrued compensation	3,612	2,725	1,395
Other liabilities, current	(63)	23	19
Other liabilities, noncurrent	376	(60)	(25)
Net cash used in operating activities	(93,022)	(67,723)	(30,890)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(3,101)	(1,352)	(993)
Purchases of marketable securities	(127,724)	(105,500)
Maturities of marketable securities	96,009
Increase in restricted cash	(915)
Purchases of other assets	(175)
Net cash used in investing activities	(35,906)	(106,852)	(993)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of issuance costs	231,955	116,995	126,230
Proceeds from sale of restricted stock			2,108
Proceeds from issuance of common stock in settlement of employee stock purchase plan and exercise of stock options	3,892	1,240	45
Repurchases of unvested restricted stock purchases	(421)	(90)	(67)
Taxes paid related to net shares settlement of equity awards	(238)
Net cash provided by financing activities	235,188	118,145	128,316
Net increase (decrease) in cash and cash equivalents	106,260	(56,430)	96,433
Cash and cash equivalents at beginning of period	92,072	148,502	52,069
Cash and cash equivalents at end of period	198,332	92,072	148,502
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING INFORMATION:
Leasehold improvements paid for by landlord	11,086
Accretion of Series A redeemable convertible preferred stock			$ 4,180
Accrued purchase of property and equipment	$ 1,536	$ 114

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation Global Blood Therapeutics Inc. (the “Company”, “we”, “us”, and “our”) was incorporated in Delaware in February 2011 and commenced operations in May 2012. We are a clinical-stage biopharmaceutical company determined to discover, develop and deliver innovative treatments that provide hope to underserved patient communities. Our primary activities have been establishing our facilities, recruiting personnel, conducting development of our product candidates, including clinical trials, and raising capital. Our principal operations are based in South San Francisco, California, and we operate in one segment. Initial Public Offering In August 2015, we closed our initial public offering (“IPO”), at which time we issued 6,900,000 shares of our common stock at a price of $20.00 per share. We received $126.2 million, net of underwriting discounts and commissions, and offering expenses incurred by us. Upon the closing of our IPO, all outstanding shares of our redeemable convertible preferred stock converted by their terms into 19,746,614 shares of common stock. Follow-on Offerings In February 2017, we completed a follow-on offering and issued an aggregate of 5,867,347 shares of our common stock at a price of $24.50 per share, including 765,306 shares of our common stock sold directly to the underwriters when they exercised their over-allotment option at the price of $24.50 per share. We received total proceeds of approximately $135.6 million from the offering, net of underwriting discounts and commissions, and offering expenses. In December 2017, we completed a follow-on offering and issued 2,631,579 shares of common stock at a price of $38.00 per share with proceeds of $96.4 million net of underwriting costs and commissions, and estimated offering expenses. In addition, in January 2018, we sold an additional 394,736 shares of our common stock directly to the underwriters when they exercised their over-allotment option at the price of $38.00 per share for proceeds of $14.5 million net of underwriting costs and commissions, which was not reflected on the consolidated balance sheets as of December 31, 2017. Common Stock Sales Agreement In August 2017, we entered into a sales agreement with Cowen and Company, LLC (“Cowen”) to create an at-the-market equity program, under which we may issue and sell from time to time shares of our common stock having an aggregate offering price of up to $125.0 million through Cowen acting as our sales agent and/or principal. Upon delivery of a placement notice and subject to the terms and conditions of the sales agreement, Cowen will use its commercially reasonable efforts to sell the shares from time to time, based upon our instructions. Sales of our common stock, if any, will be made at market prices by any method that is deemed to be an “at the market offering” as defined in Rule 415 under the Securities Act of 1933, as amended, including sales made directly on the NASDAQ Stock Market or any other existing trading market for the common stock, at market prices or as otherwise agreed with Cowen. Under the sales agreement, Cowen will be entitled to a commission of up to 3.0% of the gross proceeds per share sold. We have no obligation to sell any shares under the sales agreement, and may at any time suspend offers under the sales agreement or terminate the sales agreement by giving written notice as specified in the sales agreement. As of the date of filing of this annual report on Form 10-K, we have not sold any shares of common stock under the sales agreement. Need for Additional Capital In the course of our development activities, we have sustained operating losses and we expect such losses to continue over the next several years. Our ultimate success depends on the outcome of our research and development activities. As of December 31, 2017, we had an accumulated deficit of $298.0 million. We expect to incur additional losses in the future to conduct product research and development and we anticipate the need to raise additional capital to fully implement our business plan. We intend to raise such capital through the issuance of additional equity, potentially through borrowings, and strategic alliances with partner companies. However, if such financing is not available at adequate levels or when it will be required, we will need to reevaluate our operating plans. We believe that our existing capital resources consisting of cash and cash equivalent and marketable securities will be sufficient to fund our operations for at least the next twelve months from the date of issuance of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Reclassification Certain prior year amounts in the consolidated statements of operations and comprehensive loss have been reclassified to conform to the current year’s presentation. The results of the reclassification had no impact on the previously reported results of operations or financial condition as of December 31, 2016. Use of Estimates The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires us 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 consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions. Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany transactions and balances have been eliminated upon consolidation. Segment Reporting We have determined that we operate in a single segment based upon the way the business is organized for making operating decisions and assessing performance. The Company has only one operating segment related to the development of pharmaceutical products. All property and equipment is maintained in the United States. Cash and Cash Equivalents We consider all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value. Concentration of Credit Risk We invest in a variety of financial instruments and, by our policy, limit the amount of credit exposure with any one issuer, industry or geographic area for investments other than instruments backed by the U.S. federal government. Investments in Marketable Securities We invest in marketable securities, primarily money market funds, corporate debt securities, government securities, government agency securities, and certificates of deposits. We classify our marketable securities as available-for-sale securities and report them at fair value in cash equivalents or marketable securities on the consolidated balance sheets with related unrealized gains and losses included within accumulated other comprehensive income (loss) on the consolidated balance sheet. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in interest and other income (loss). The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income. We regularly review all of our investments for other-than-temporary declines in estimated fair value. Our review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. When we determine that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, we reduce the carrying value of the security and record a loss for the amount of such decline. Fair Value Measurement The carrying amounts of certain financial instruments, including cash and cash equivalents, other receivables as included in other assets, current, restricted cash, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Property and Equipment, Net Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the assets, three years for computer equipment and five years for laboratory equipment. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged as expense in the statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations. Impairment of Long-Lived Assets We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. There have been no impairments of our long-lived assets for the periods presented. Restricted Cash Restricted cash consists of cash deposits held by our financial institution as collateral for our letter of credit under our facility lease. Accruals of Research and Development Costs We record accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development. These costs are a significant component of our research and development expenses. A substantial portion of our ongoing research and development activities are conducted by third-party service providers, including contract research organizations and contract manufacturing organizations. We accrue the costs incurred under our agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. We determine the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. We make significant judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, we adjust our accruals. We have not experienced any material deviations between accrued clinical trial expenses and actual clinical trial expenses. However, actual services performed, number of subjects enrolled, and the rate of subject enrollment may vary from our estimates, resulting in adjustments to clinical trial expense in future periods. Changes in these estimates that result in material changes to our accruals could materially affect our results of operations. Leases We enter into lease agreements for our office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the noncancelable term of the lease and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability, which is included within other liabilities on the consolidated balance sheet. Incentives granted under our facilities leases, including rent holiday and allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the noncancelable term of the lease. Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. Our comprehensive income (loss) is comprised of net loss and changes in unrealized gains and losses on our marketable securities. Research and Development Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on our behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are capitalized and then expensed as the related goods are delivered or the services are performed. Stock-Based Compensation We measure and recognize stock-based compensation expense, including employee and non-employee equity awards, based on fair value at the grant date. We use the Black-Scholes option-pricing model to calculate fair value. Stock-based compensation expense recognized in the consolidated statements of operations is based on stock awards ultimately vested, taking into consideration actual forfeitures. For options granted to non-employees, we revalue the unearned portion of the stock-based compensation and the resulting change in fair value is recognized in the consolidated statements of operations over the period the related services are rendered. Income Taxes We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. We recognize benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. To date, there have been no interest or penalties incurred in relation to the unrecognized tax benefits. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given our net loss. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases. 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. 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. The standard is effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted. In January 2018, the FASB issued a proposed amendment to ASU 842 that would provide an entity the optional transition method to initially account for the impact of the adoption with a cumulative adjustment to accumulated deficit on the effective date of the ASU, January 1, 2019 rather than January 1, 2017, which would eliminate the need to restate amounts presented prior to January 1, 2019. ASU 842 provides practical expedients that allow entities to not (i) reassess whether any expired or existing contracts are considered or contain leases; (ii) reassess the lease classification for any expired or existing leases; and (iii) reassess initial direct costs for any existing leases. We are in the process of analyzing initial data gathered to evaluate the impact of adopting the ASU on its consolidated financial statements and the related systems required to capture the increased reporting and disclosures associated with the ASU. We plan to adopt the standard on January 1, 2019 and expect to elect the use of practical expedients. If the proposed amendment to ASU 842 is adopted, we plan to elect the transition method for adoption as described above. In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments. The new standard provides guidance on eight specific cash flow classification issues. The standard is effective for annual periods beginning after December 15, 2017, and interim periods within those annual periods. We believe that the adoption of this new standard will have no impact on our financial position or results of operations and have not elected to early adopt the amendment. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash. The new standard requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The standard is effective for annual periods beginning after December 15, 2017, and interim periods within those annual periods. We believe that the adoption of this new standard will have no material impact on our financial position or results of operations and have not elected to early adopt the amendment. In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) Accounting Pronouncements Adopted In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting The impact of adopting ASU 2016-09 resulted in the following: • Classification of excess income tax benefits from stock-based compensation arrangement as a discrete item within income tax expense, rather than recognizing such excess income tax benefits in additional paid-in capital. The adoption of this guidance resulted in an increase of approximately $1.1 million of net operating losses, which has an impact of $0.4 million on our deferred tax assets before our full valuation allowance established against the related deferred tax assets. • Recognition of forfeitures as they occur. The cumulative effect adjustment as a result of the adoption of ASU 2016-09 on a modified retrospective basis was insignificant. • A change in classification of cash flows resulting from excess tax benefits and cash paid by us when directly withholding shares for tax-withholding purposes on a retrospective basis. The adoption of these provisions did not result in changes in our condensed consolidated statements of cash flow. There were no other material impacts to our condensed consolidated financial statements as a result of adopting this updated standard.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Fair value accounting is applied for all financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). Our financial instruments consist of cash and cash equivalents, marketable securities, other receivables as included in other assets, current, restricted cash, accounts payable and accrued liabilities. Cash and cash equivalents, marketable securities and restricted cash are reported at their respective fair values on our Consolidated Balance Sheets. The remaining financial instruments are reported on our Consolidated Balance Sheets at cost that approximate current fair values due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the consolidated balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1— Level 2— Level 3— The following table summarizes our financial assets measured at fair value on a recurring basis (in thousands): December 31, 2017 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 134,744 $ 134,744 $ — $ — Corporate debt securities 46,977 — 46,977 — U.S. government agency securities 54,989 — 54,989 — Certificates of deposits 9,129 — 9,129 — U.S. government securities 20,007 — 20,007 — Total financial assets $ 265,846 $ 134,744 $ 131,102 $ — December 31, 2016 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 72,597 $ 72,597 $ — $ — Corporate debt securities 48,531 — 48,531 — U.S. government agency securities 39,712 — 39,712 — Certificates of deposits 19,117 — 19,117 — U.S. government securities 7,999 — 7,999 — Total financial assets $ 187,956 $ 72,597 $ 115,359 $ — We estimate the fair values of our investments in corporate debt securities, government and government related securities and certificates of deposits by taking into consideration valuations obtained from third-party pricing services. The fair value of our marketable securities classified within Level 2 is based upon observable inputs that may include benchmark yields, reported trades, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including market research publications. At December 31, 2017, the weighted average remaining contractual maturities of our Level 2 investments was less than one year and all of these investments are rated A-1/P-1/F1 or A/A2, or higher by Moody’s, S&P and Fitch. There were no transfers between Level 1 and Level 2 during the periods presented.

Available-for-Sale Securities

Available-for-Sale Securities	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Securities	4. Available-for-Sale Securities Estimated fair values of available-for-sale securities are generally based on prices obtained from commercial pricing services. The following table is a summary of available-for-sale securities recorded in cash and cash equivalents, restricted cash, or marketable securities in our Consolidated Balance Sheets (in thousands): December 31, 2017 December 31, 2016 Amortized Unrealized Unrealized Estimated Fair Amortized Unrealized Unrealized Estimated Fair Financial Assets: Money market funds $ 134,744 $ — $ — $ 134,744 $ 72,597 $ — $ — $ 72,597 Corporate debt securities 47,108 — (131 ) 46,977 48,594 2 (65 ) 48,531 U.S. government agency securities 55,170 — (181 ) 54,989 39,763 1 (52 ) 39,712 Certificates of deposits 9,142 — (13 ) 9,129 19,169 — (52 ) 19,117 U.S. government securities 20,018 — (11 ) 20,007 7,999 — — 7,999 Total $ 266,182 $ — $ (336 ) $ 265,846 $ 188,122 $ 3 $ (169 ) $ 187,956 The following table summarizes the classification of the available-for-sale securities on our Consolidated Balance Sheets (in thousands): December 31, 2017 December 31, 2016 Cash and cash equivalents $ 134,746 $ 82,696 Short-term marketable securities 116,493 55,202 Long-term marketable securities 14,607 50,058 Total $ 265,846 $ 187,956 We do not intend to sell the investments that are in an unrealized loss position, and it is unlikely that we will be required to sell the investments before recovery of their amortized cost basis, which may be maturity. We have determined that the gross unrealized losses on our marketable securities at December 31, 2017 were temporary in nature. All unrealized losses from all marketable securities at December 31, 2017 are not material.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and Equipment Property and equipment consist of the following (in thousands): December 31, 2017 2016 Laboratory equipment $ 5,715 $ 3,895 Computer equipment 1,594 972 Leasehold improvements 12,642 678 Construction-in-progress 419 137 Total property and equipment 20,370 5,682 Less: accumulated depreciation and amortization (3,799 ) (3,262 ) Property and equipment, net $ 16,571 $ 2,420 Depreciation expense was $1.7 million for the year ended December 31, 2017, $1.2 million for the year ended December 31, 2016 and $0.9 million for the year ended December 31, 2015. Accrued Liabilities Accrued liabilities consist of the following (in thousands): December 31, 2017 2016 Accrued clinical and manufacturing expenses $ 8,035 $ 4,759 Accrued professional and consulting services 1,007 507 Other 1,093 53 Total accrued liabilities $ 10,135 $ 5,319 Other liabilities, current and noncurrent Other liabilities consist of the following (in thousands): December 31, 2017 2016 Restricted shares subject to repurchase, current $ 373 $ 846 Deferred rent, current — 61 Other payable — 2 Total other liabilities, current $ 373 $ 909 Restricted shares subject to repurchase, noncurrent $ 161 $ 534 Deferred rent, noncurrent 11,491 29 Total other liabilities, noncurrent $ 11,652 $ 563

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Common Stock Reserved for Issuance We have reserved sufficient shares of common stock for issuance upon the exercise of stock options, vesting of restricted stock units and restricted shares subject to future vesting. Common stockholders are entitled to dividends if and when declared by the board of directors, subject to the prior rights of any preferred stockholders. As of December 31, 2017, no common stock dividends had been declared by the board of directors. We have reserved shares of common stock, on an as-converted basis, for future issuance as follows: December 31, 2017 2016 Restricted shares subject to future vesting 241,617 672,112 Restricted stock units 820,713 — Options issued and outstanding 2,945,901 2,769,702 Shares available for future grant under the 2015 Plan and 2017 Inducement Equity Plan 1,708,680 1,401,153 Employee stock purchase plan 186,033 76,118 Total 5,902,944 4,919,085 Restricted Stock We have issued restricted stock awards to employees under our 2012 Stock Option and Grant Plan (the “2012 Plan”). Under the related stock purchase agreements, we have the right to repurchase the common stock at the lower of fair market value and the stockholders’ original purchase price which right lapses according to individual vesting schedules. In order to vest, the holders are required to provide continued service to us. Upon vesting, the appropriate amounts are transferred from liabilities to additional paid-in capital. If the employment or other service relationship of the holder of any unvested restricted common stock is terminated for any reason, we have the right to repurchase the unvested shares at the lower of fair market value or the stockholder’s original purchase price. As such, the shares subject to future vesting are not deemed outstanding for accounting purposes until the shares vest. Restricted shares subject to repurchase were as follows: December 31, 2017 2016 Restricted shares subject to repurchase: Shares issued pursuant to the 2012 Stock Option and Grant Plan 241,617 672,112 Total restricted shares subject to repurchase 241,617 672,112

Share-based Compensation

Share-based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation	7. Share-based Compensation 2017 Inducement Equity Plan In January 2017, we adopted the 2017 Inducement Equity Plan (the “2017 Inducement Plan”). Under the 2017 Inducement Plan, 300,000 shares of our common stock were initially reserved for the issuance of non-qualified stock options and other equity-based awards to induce highly-qualified prospective officers and employees who are not currently employed by us or our subsidiaries to become employed with our company. The number of shares initially reserved for grant is subject to adjustment for reorganization, recapitalization, stock dividend, stock split, or similar changes in our capital stock. As of December 31, 2017, there were 170,000 shares reserved for the future issuance of equity awards under the 2017 Inducement Plan. 2015 Stock Option and Incentive Plan In July 2015, we adopted the 2015 Stock Option and Incentive Plan (the “2015 Plan”). Under the 2015 Plan, 1,430,000 shares of our common stock were initially reserved for the issuance of stock options, restricted stock, and other equity-based awards to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by our stockholders at inception. Awards granted under the 2015 Plan expire no later than 10 years from the date of grant. For incentive stock options and non-statutory stock options, the option price shall not be less than 100% of the fair market value on the day of grant. If at the time we grant an option and the optionee directly or by attribution owns stock possessing more than 10% of the total combined voting power of all our classes of stock, the option price is required to be at least 110% of the fair market value on the day of grant. Options granted typically vest over a 4-year period but may be granted with different vesting terms. 2012 Stock Option and Grant Plan In 2012, the Company adopted the 2012 Stock Option and Grant Plan (the “2012 Plan”) under which our Board of Directors was authorized to grant incentive stock options to employees, including officers and members of the Board of Directors who are also employees of ours, and non-statutory stock options (options that do not qualify as incentive options) and/or our restricted stock and other equity-based awards to employees, officers, directors, or consultants of ours. Previously, we had initially reserved 2,785,713 shares of common stock for issuance under the 2012 Plan. On April 9, 2015 we increased the number of shares available under the 2012 Plan by 1,000,000 to a total of 3,785,713 shares. Awards granted under the 2012 Plan expire no later than 10 years from the date of grant. Upon adoption of the 2015 Plan, no new awards or grants are permitted under the 2012 Plan. Stock Option Activity The following table summarizes activity under the Company’s stock option plans, including the 2017 Inducement Plan, 2015 Plan and the 2012 Plan and related information (in thousands, except share and per share amounts and term): Number Weighted- Weighted- Aggregate Outstanding — December 31, 2016 2,769,702 $ 11.99 7.81 Options granted 1,001,990 25.08 Options exercised (578,455) 4.87 Options canceled (247,336) 16.12 Outstanding — December 31, 2017 2,945,901 $ 17.50 8.25 $ 65,041 Vested and exercisable — December 31, 2017 1,259,657 $ 13.91 7.74 $ 32,427 Vested and expected to vest — December 31, 2017 2,945,901 $ 17.50 8.25 $ 65,041 The aggregate intrinsic values of options outstanding, vested and exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value our common stock as of December 31, 2017. The total intrinsic value of options exercised was $11.3 million for the year ended December 31, 2017, $2.4 million for the year ended December 31, 2016 and $0.4 million for the year ended December 31, 2015. The weighted-average estimated fair value of stock options granted was $16.19 for the year ended December 31, 2017, $11.74 for the year ended December 31, 2016 and $8.56 for the year ended December 31, 2015. Stock Options Granted to Employees with Service-based Vesting Valuation Assumptions The fair values of stock options granted to employees were calculated using the following assumptions: Year Ended December 31, 2017 2016 2015 Expected term (in years) 5.3-6.1 5.3-6.1 5.3-6.3 Volatility 68.9%-75.6% 70.6%-82.3% 73.8%-87.6% Risk-free interest rate 1.8%-2.3% 1.1%-2.1% 1.5%-1.8% Dividend yield — — — The fair value of the shares of common stock underlying stock options before our IPO was determined by our Board of Directors. Prior to our IPO in August 2015, because there was no public market for our common stock, the Board of Directors determined fair value of the common stock at the time of grant of the option by considering a number of objective and subjective factors including important developments in our operations, valuations performed by an independent third party, sales of convertible preferred stock, actual operating results and financial performance, the conditions in the biotechnology industry and the economy in general, the stock price performance and volatility of comparable public companies, and the lack of liquidity of our common stock, among other factors. In determining the fair value of the options granted, we used the Black-Scholes-Merton option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment to determine. Expected Term Expected Volatility Risk-Free Interest Rate Expected Dividend Stock Options Granted to Non-employees with Service-Based Vesting Valuation Assumptions Stock-based compensation related to stock options granted to non-employees is recognized as the stock options are earned. There are no stock options granted to non-employees outstanding as of December 31, 2017. The fair value of the stock options granted during years ended December 31, 2016 and December 31, 2015 is calculated at each reporting date using the Black-Scholes option-pricing model with the following assumptions: Year Ended December 31, 2016 2015 Expected term (in years) 9.5 4.0-9.9 Volatility 78.0% 73.4%-91.2% Risk-free interest rate 1.74% 0.8-2.7% Dividend yield — — Common Stock Issued for License Agreement In September 2015, we executed an agreement with the Regents of the University of California, or the Regents, for an exclusive license to those rights the Regents may own in certain patents and patent applications relating to voxelotor and voxelotor analogs, and in exchange have committed to pay a royalty of less than 1% on future net sales. In connection with this agreement we issued 85,714 shares of our common stock with an estimated fair value of $4.5 million, which was recorded in research and development expense in our consolidated statements of operations. Restricted Stock Units In January 2017, the Compensation Committee of our Board of Directors approved the commencement of granting restricted stock units (“RSUs”) to our employees. RSUs are share awards that entitle the holder to receive freely tradable shares of our common stock upon the completion of a specific period of continued service. RSUs are generally subject to forfeiture if employment terminates prior to the release of vesting restrictions. RSUs granted are valued at the market price of our common stock on the date of grant. We recognize noncash compensation expense for the fair value of RSUs on a straight-line basis over the requisite service period of these awards. The following table summarizes activity of RSUs granted to employees with service-based vesting under the 2017 Inducement Plan and 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested units — December 31, 2016 — $ — — $ — RSUs granted 520,940 24.74 RSUs vested (42,679 ) 21.48 RSUs forfeited (10,798 ) 29.24 Non-vested units — December 31, 2017 467,463 $ 24.93 1.71 $ 18,395 Restricted Stock Purchases When Restricted Stock Purchases (“RSPs”) are granted, the individual purchases the shares at the grant date fair value of the underlying common stock. The purchase of the stock is subject to forfeiture prior to vesting at the lower of fair value and the original purchase price. The award is treated similarly to an early exercise of stock options for accounting purposes. A summary of our unvested restricted stock for the year ended December 31, 2017 is as follows: Number of RSPs Weighted Average Outstanding — December 31, 2016 672,112 $ 1.10 RSPs vested (306,389 ) 0.90 Repurchased by Company (124,106 ) 0.84 Outstanding — December 31, 2017 241,617 $ 1.46 Restricted Stock Purchases with Service-Based Vesting RSPs granted during the years ended December 31, 2015 generally vest over four years, subject to the individual holder’s continued service relationship with us. There were no RSPs granted during the years ended December 31, 2017 and 2016. The restricted common stock granted to an employee is valued using the Black-Scholes-Merton option-pricing model based on the common stock fair value at the time of the grant. For restricted common stock issued to consultants, we remeasure the fair value of the restricted shares as they vest at each reporting period using the Black-Scholes-Merton option-pricing model reflecting the remaining vesting period. Performance-Contingent Awards Granted to Employees On April 9, 2015, our Board of Directors granted a total of 326,424 performance-contingent awards to members of our senior management team. Of the total performance-contingent awards granted, 227,139 were performance-contingent options and 99,285 were performance-contingent shares of restricted common stock. The exercise price of each performance-contingent options and the purchase price for the performance-contingent restricted shares is $3.40 per share, which the Board of Directors determined was the fair market value on the grant date. The awards had dual triggers of vesting based upon the successful achievement of four corporate operating milestones within specified timelines, as well as a requirement for continued employment. During the year ended December 31, 2016, the Compensation Committee of our Board of Directors modified one of the corporate operating milestones, which resulted in two of the corporate operating milestones being achieved; accordingly, an aggregate of 94,502 shares underlying options and 49,643 shares of restricted stock associated with these two milestones vested. One of the remaining two corporate operating milestones was not met within the timeframe required for achievement during the year ended December 31, 2016; accordingly, 47,500 shares underlying options were forfeited and 24,821 shares of restricted stock associated with the milestone were repurchased by us upon forfeiture. During the quarter ended September 30, 2017, the final remaining corporate milestone was not met within the timeframe required for achievement; accordingly, 30,710 shares underlying options were forfeited and 24,821 shares of restricted stock associated with the milestone were repurchased by us. Market-Condition Awards Granted to Employees On August 11, 2017, our Board of Directors approved awards up to an aggregate of 365,250 RSUs to certain of our senior management team under the 2015 Plan, the vesting of which are contingent upon a combination of continued employment and achieving certain market capitalization milestones. The market-condition awards do not vest until the achievement of their respective market capitalization milestones, which must occur on or before December 31, 2019. The grant date fair value of these market-condition awards was estimated using a Monte Carlo simulation model. The derived service periods, which are the estimated periods of time that would be required to satisfy the market conditions, are also determined at the grant date. We record expense on a straight-line basis over the applicable derived service periods. The following table summarizes activity of the market-condition awards under the 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested market-condition awards — December 31, 2016 — $ — — Granted 365,250 15.15 Vested — — Forfeited (12,000 ) 15.15 Non-vested market-condition awards — December 31, 2017 353,250 $ 15.15 0.73 $ 13,900 The following table summarizes the assumptions used to estimate the fair value of the market-condition awards during the year ended December 31, 2017: Valuation date stock price $ 28.55 Volatility 65.6 % Risk-free interest rate 1.4 % Dividend yield — At December 31, 2017, total unrecognized compensation expense related to unvested market-condition awards was $3.2 million, which is expected to be recognized over their respective remaining derived service periods. The weighted average derived service period is 1.12 years. For the year ended December 31, 2017, we recognized $2.2 million in stock-based compensation expense related to the market-condition awards. On April 9, 2015, our Board of Directors granted a market-condition award to our Chief Executive Officer of 99,285 shares of restricted common stock under the 2012 Plan, with a purchase price of $3.40 per share, which the Board of Directors determined was the fair market value on the grant date. The market-condition award did not vest until our market capitalization (determined based on the number of shares of common stock outstanding multiplied by the closing market price for our common stock as reported on NASDAQ) exceeded at least $2.0 billion for 20 consecutive trading days on or before the date twenty-four (24) months after the closing of our IPO. During the year ended December 31, 2017, the market capitalization goal was not met within the timeframe required for achievement; accordingly, 99,285 shares of restricted stock associated with the market condition award were repurchased by us. The fair value of the market-condition award of $0.70 was determined on the grant date utilizing a lattice model with an expected term of 2.4 years. In August 2015, we began to recognize compensation costs for this award concurrent with the closing of our IPO. The compensation cost for the awards with a market condition is not reversed when the market condition is not satisfied. Employee Stock Purchase Plan In July 2015, we adopted the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). Under the 2015 ESPP our employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2015 ESPP provides for offering periods of six months in duration. The purchase periods end on either January 31st or July 31st. Contributions under the 2015 ESPP are limited to a maximum of 15% of an employee’s eligible compensation. ESPP purchases are settled with common stock from the ESPP’s previously authorized and available pool of shares. During 2017, 76,585 shares were issued under the ESPP for $1.0 million. The fair values of the rights granted under the 2015 ESPP were calculated using the following assumptions: Year Ended Year Ended Expected term (in years) 0.5 0.5 Volatility 60.1-63.5 % 83.1-84.6 % Risk-free interest rate 0.7-1.2 % 0.4-0.5 % Dividend yield — % — % Stock-Based Compensation Expense Total stock-based compensation recognized by our research and development function and our general and administrative function was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Research and development $ 5,905 $ 4,153 $ 2,031 General and administrative 7,777 5,082 1,192 Total stock-based compensation expense $ 13,682 $ 9,235 $ 3,223 During the year ended December 31, 2016, we recorded charges of $1.5 million relating to the fair value of stock options which were modified for two terminated employees. $0.9 million of these charges were classified as research and development expenses and the remaining $0.6 million of these charges were classified as general and administrative expenses. Total stock-based compensation recognized by employees and non-employees was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Employee $ 13,682 $ 9,003 $ 2,359 Non-employee — 232 864 Total stock-based compensation expense $ 13,682 $ 9,235 $ 3,223 Unrecognized Stock-Based Compensation Expense and Weighted-Average Remaining Amortization Period As of December 31, 2017 the unrecognized stock-based compensation cost, net of expected forfeitures, and the estimated weighted-average amortization period, using the straight-line attribution method, was as follows (in thousands, except amortization period): Unrecognized Compensation Cost Weighted-average remaining amortization period (years) Options $ 20,969 2.4 Restricted stock purchases 300 0.9 Restricted stock units 13,670 2.2 ESPP 53 0.1 Total stock-based compensation expense $ 34,992 2.3

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2017
Postemployment Benefits [Abstract]
Defined Contribution Plan	8. Defined Contribution Plan In 2013, we began to sponsor a 401(k) retirement plan, in which substantially all of our full-time employees are eligible to participate. Eligible participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. Prior to 2015, we had not provided any contributions to the plan. We made contributions to the Plan for eligible participants, and recorded contribution expenses of $0.3 million for the year ended December 31, 2017, $0.2 million for the year ended December 31, 2016 and $33,000 for the year ended December 31, 2015.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes On December 22, 2017, the President signed the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act, among other things, lowered the U.S. corporate income tax rate from 35% to 21% effective January 1, 2018. Consequently, our net deferred tax assets as of December 31, 2017 were significantly reduced to reflect the estimated impact of the Tax Act. Due to our lack of earnings history and uncertainties surrounding our ability to generate future taxable income, the net deferred tax assets have been fully offset by a valuation allowance. The significant reduction in our net deferred tax assets are fully offset by a reduction in valuation allowance, resulting in no impact to our income tax expense. The components of the loss before income taxes were as follows (in thousands): Year Ended December 31, 2017 2016 2015 Loss before provision for income taxes: United States $ (101,288 ) $ (70,103 ) $ (46,360) International (15,736 ) (12,365 ) — $ (117,024 ) $ (82,468 ) $ (46,360) No provision for income taxes was recorded for the years ended December 31, 2017, December 31, 2016 and December 31, 2015. We have incurred net operating losses for all the periods presented. We have not reflected any benefit of such net operating loss carryforwards in the accompanying consolidated financial statements. We have established a full valuation allowance against the related deferred tax assets due to the uncertainty surrounding the realization of such assets. The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2017 2016 2015 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % Non-deductible changes in fair value and other 0.7 3.9 (4.8 ) Federal and state tax credits 7.3 8.3 1.4 Change in valuation allowance (14.8 ) (41.1 ) (30.6 ) Foreign rate differential (4.6 ) (5.1 ) — Officer compensation limitation (0.9 ) — — Tax reform – tax rate change (21.7 ) 0.0 % 0.0 % Provision for Taxes 0.0 % 0.0 % 0.0 % The components of the deferred tax assets and liabilities are as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 51,448 $ 51,348 Tax credits 32,732 15,167 Property and equipment 116 9 Accruals and reserves 1,668 1,739 Stock based compensation 1,915 1,859 Gross deferred tax assets 87,879 70,122 Valuation allowance (87,879 ) (70,122 ) Realization of the deferred tax assets is dependent upon future taxable income, if any, the amount and timing of which are uncertain. We have established a valuation allowance to offset deferred tax assets as of December 31, 2017 and 2016 due to the uncertainty of realizing future tax benefits from our net operating loss carryforwards and other deferred tax assets. The valuation allowance increased approximately $17.8 million, $34.0 million, and $16.7 million during the years ended December 31, 2017, 2016, and 2015, respectively. The increase in the valuation allowance is mainly related to the increase in net operating loss carryforwards generated during the respective taxable years. At December 31, 2017, we had net operating loss carryforwards for Federal income tax purposes of $194.5 million which are available to offset future taxable income, if any, through 2036. We also had net operating loss carryforwards for state income tax purposes of $151.9 million which are available to offset future taxable income, if any. They begin to expire 2032 through 2036. As of December 31, 2017, we had research and development/orphan drug tax credit carryforwards of approximately $30.0 million and $3.4 million available to reduce future taxable income, if any, for federal and state income tax purposes, respectively. If not utilized, the federal credit carryforwards will begin expiring in 2031, and the state credits carryforward indefinitely. In general, if we experience a greater than 50 percentage point aggregate change in ownership over a three-year period (a Section 382 ownership change), utilization of our pre-change NOL carryforwards are subject to an annual limitation under Section 382 of the Internal Revenue Code (California has similar laws). The annual limitation generally is determined by multiplying the value of our stock at the time of such ownership change (subject to certain adjustments) by the applicable long-term tax-exempt rate. Such limitations may result in expiration of a portion of the NOL carryforwards before utilization. We have not utilized any NOL carryovers through December 31, 2017. In addition, our deferred tax assets are subject to full valuation allowance, and thus no benefit for deferred tax assets are recorded on our books. Our ability to use the remaining NOL carryforwards may be further limited if we experience a Section 382 ownership change as a result of future changes in our stock ownership. No liability related to uncertain tax positions is recorded on the consolidated financial statements. All uncertain tax positions are currently recorded as a reduction to our deferred tax asset. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2017 2016 Balance at beginning of year $ 5,296 $ 1,005 Additions based on tax positions related to current year 6,346 4,291 Decreased for prior period positions (492) Unrecognized tax benefit - December 31 $ 11,150 $ 5,296 We do not expect that our uncertain tax positions will materially change in the next twelve months. The reversal of the uncertain tax benefits will not impact our effective tax rate as we continue to maintain a full valuation allowance against our deferred tax assets. We file income tax returns in the United States, California and other states. We are not currently under examination by income tax authorities in federal, state or other jurisdictions. All tax returns will remain open for examination by the federal and state authorities for three and four years, respectively, from the date of utilization of any net operating loss or credits.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions Our related parties include Third Rock Ventures, LLC (“TRV”) and one member of our Board of Directors is also a partner in TRV. Management and advisory fee expense incurred with TRV were zero for the years ended December 31, 2017 and 2016, and $65,000 for the year ended December 31, 2015 for services which we requested from TRV. We had no outstanding payable to TRV as of December 31, 2017 and December 31, 2016.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Facilities In March 2017, we entered into a noncancelable operating lease (the “Lease”) for approximately 67,185 square feet of space in South San Francisco, California (the “New Facility”). The date on which we became responsible for paying rent under the Lease was December 15, 2017 (the “Rent Commencement Date”). The Lease expires 10 years after the Rent Commencement Date. The Lease grants us an option to extend the Lease for an additional 10-year period. Future minimum rental payments under the Lease during the 10-year term are $48.3 million in the aggregate. The Lease further provides that we are obligated to pay to the landlord certain costs, including taxes and operating expenses. The Lease term commenced in November 2017 as we gained control over physical access to the New Facility. We have capitalized $12.4 million of costs in leasehold improvements, as included in property and equipment, net, for construction of leasehold improvements at the New Facility as of December 31, 2017. These leasehold improvements were acquired with the tenant inducement of $11.1 million provided under the Lease and our capital resources of $1.3 million. We provided a standby letter of credit of $0.9 million as security for our obligations under the Lease which was issued in March 2017. This standby letter of credit is classified as restricted cash. Previously, we leased approximately 36,740 square feet of office and lab space located in South San Francisco, California (the “Former Facility”). All of our lease agreements associated with the Former Facility expire on or before April 2018. We terminated one of the lease agreements for our Former Facility prior to its expiration date without incurring termination cost. The remainder of the lease agreements associated with our Former Facility expired by December 31, 2017. We accelerated depreciation related to the leasehold improvements and certain assets that remained at the Former Facility. The incremental depreciation associated with acceleration of the useful life was $0.1 million for the year ended December 31, 2017. Future aggregate minimum lease payments under the noncancelable operating leases are as follows (in thousands): Year ending December 31, Amount 2018 3,265 2019 4,400 2020 4,545 2021 4,694 Thereafter 31,360 Total $ 48,264 Through February 2015, we were a party to a Space Sharing Agreement and a Shared Services Agreement with a biotechnology company that was also majority-owned by TRV. Under these agreements, specified expenses were shared equally between the two companies at cost and not subject to any markup or markdown. Under these agreements, we recorded reimbursements of $33,000 for the year ended December 31, 2015. Rent expense for the facility operating leases consisted of the following (in thousands): Year Ended December 31, 2017 2016 2015 Minimum rental $ 2,001 $ 1,272 $ 971 Net reimbursement under Space Sharing Agreement — — (10 ) Facility rental expense, net $ 2,001 $ 1,272 $ 961 Indemnifications We indemnify each of our directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at our request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, we currently hold director liability insurance. This insurance allows the transfer of risk associated with our exposure and may enable us to recover a portion of any future amounts paid. We believe that the fair value of these indemnification obligations is minimal. Accordingly, we have not recognized any liabilities relating to these obligations for any period presented. Other Liabilities for loss contingencies arising from claims, assessments, litigation, fines, and penalties and other sources are recorded when it is probable that a liability has been incurred and the amount can be reasonably estimated. Legal costs incurred in connection with loss contingencies are expensed as incurred. As of December 31, 2017, we have not recognized any liabilities for loss contingencies. We are not a party to any current litigation.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss per Share	12. Net Loss per Share Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding for the period. Since we were in a loss position for all periods presented, diluted net loss per share is the same as basic net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive. The following securities were not included in the diluted net loss per share calculations because their effect was anti-dilutive: December 31, 2017 2016 2015 Options to purchase common stock 2,945,901 2,769,702 2,058,787 Restricted stock subject to future vesting 241,617 672,112 1,097,288 Restricted stock units 820,713 — — Common stock potentially issuable for ESPP purchases — 8,386 9,491 Total 4,008,231 3,450,200 3,165,566

Selected Quarterly Financial In

Selected Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information (Unaudited)	Selected Quarterly Financial Information (unaudited) The following table provides the selected consolidated quarterly financial data for 2017 and 2016: Quarter Ended (in thousands, except per share amounts) December 2017 September 30, 2017 June 30, 2017 March 31, 2017 December 31, 2016 September 30, 2016 June 30, 2016 March 31, 2016 Loss from operations $ (41,914 ) $ (29,180 ) $ (24,430 ) $ (23,721 ) $ (27,460 ) $ (21,144 ) $ (17,806 ) $ (16,717 ) Net loss $ (41,252 ) $ (28,557 ) $ (23,883 ) $ (23,332 ) $ (27,208 ) $ (20,985 ) $ (17,675 ) $ (16,600 ) Basic and diluted net loss per common share $ (0.94 ) $ (0.66 ) $ (0.55 ) $ (0.60 ) $ (0.74 ) $ (0.58 ) $ (0.58 ) $ (0.56 )

Summary of Significant Accoun20

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”).
Reclassification	Reclassification Certain prior year amounts in the consolidated statements of operations and comprehensive loss have been reclassified to conform to the current year’s presentation. The results of the reclassification had no impact on the previously reported results of operations or financial condition as of December 31, 2016.
Use of Estimates	Use of Estimates The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires us 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 consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions.
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany transactions and balances have been eliminated upon consolidation.
Segment Reporting	Segment Reporting We have determined that we operate in a single segment based upon the way the business is organized for making operating decisions and assessing performance. The Company has only one operating segment related to the development of pharmaceutical products. All property and equipment is maintained in the United States.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value.
Concentration of Credit Risk	Concentration of Credit Risk We invest in a variety of financial instruments and, by our policy, limit the amount of credit exposure with any one issuer, industry or geographic area for investments other than instruments backed by the U.S. federal government.
Investments in Marketable Securities	Investments in Marketable Securities We invest in marketable securities, primarily money market funds, corporate debt securities, government securities, government agency securities, and certificates of deposits. We classify our marketable securities as available-for-sale securities and report them at fair value in cash equivalents or marketable securities on the consolidated balance sheets with related unrealized gains and losses included within accumulated other comprehensive income (loss) on the consolidated balance sheet. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in interest and other income (loss). The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income. We regularly review all of our investments for other-than-temporary declines in estimated fair value. Our review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. When we determine that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, we reduce the carrying value of the security and record a loss for the amount of such decline.
Fair Value Measurement	Fair Value Measurement The carrying amounts of certain financial instruments, including cash and cash equivalents, other receivables as included in other assets, current, restricted cash, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the assets, three years for computer equipment and five years for laboratory equipment. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged as expense in the statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. There have been no impairments of our long-lived assets for the periods presented.
Restricted Cash	Restricted Cash Restricted cash consists of cash deposits held by our financial institution as collateral for our letter of credit under our facility lease.
Accruals of Research and Development Costs	Accruals of Research and Development Costs We record accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development. These costs are a significant component of our research and development expenses. A substantial portion of our ongoing research and development activities are conducted by third-party service providers, including contract research organizations and contract manufacturing organizations. We accrue the costs incurred under our agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. We determine the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. We make significant judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, we adjust our accruals. We have not experienced any material deviations between accrued clinical trial expenses and actual clinical trial expenses. However, actual services performed, number of subjects enrolled, and the rate of subject enrollment may vary from our estimates, resulting in adjustments to clinical trial expense in future periods. Changes in these estimates that result in material changes to our accruals could materially affect our results of operations.
Leases	Leases We enter into lease agreements for our office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the noncancelable term of the lease and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability, which is included within other liabilities on the consolidated balance sheet. Incentives granted under our facilities leases, including rent holiday and allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the noncancelable term of the lease.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. Our comprehensive income (loss) is comprised of net loss and changes in unrealized gains and losses on our marketable securities.
Research and Development	Research and Development Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on our behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are capitalized and then expensed as the related goods are delivered or the services are performed.
Stock-based Compensation	Stock-Based Compensation We measure and recognize stock-based compensation expense, including employee and non-employee equity awards, based on fair value at the grant date. We use the Black-Scholes option-pricing model to calculate fair value. Stock-based compensation expense recognized in the consolidated statements of operations is based on stock awards ultimately vested, taking into consideration actual forfeitures. For options granted to non-employees, we revalue the unearned portion of the stock-based compensation and the resulting change in fair value is recognized in the consolidated statements of operations over the period the related services are rendered.
Income Taxes	Income Taxes We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. We recognize benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. To date, there have been no interest or penalties incurred in relation to the unrecognized tax benefits.
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given our net loss.
Recent Accounting Pronouncements/Accounting Pronouncements Adopted	Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases. 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. 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. The standard is effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted. In January 2018, the FASB issued a proposed amendment to ASU 842 that would provide an entity the optional transition method to initially account for the impact of the adoption with a cumulative adjustment to accumulated deficit on the effective date of the ASU, January 1, 2019 rather than January 1, 2017, which would eliminate the need to restate amounts presented prior to January 1, 2019. ASU 842 provides practical expedients that allow entities to not (i) reassess whether any expired or existing contracts are considered or contain leases; (ii) reassess the lease classification for any expired or existing leases; and (iii) reassess initial direct costs for any existing leases. We are in the process of analyzing initial data gathered to evaluate the impact of adopting the ASU on its consolidated financial statements and the related systems required to capture the increased reporting and disclosures associated with the ASU. We plan to adopt the standard on January 1, 2019 and expect to elect the use of practical expedients. If the proposed amendment to ASU 842 is adopted, we plan to elect the transition method for adoption as described above. In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments. The new standard provides guidance on eight specific cash flow classification issues. The standard is effective for annual periods beginning after December 15, 2017, and interim periods within those annual periods. We believe that the adoption of this new standard will have no impact on our financial position or results of operations and have not elected to early adopt the amendment. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash. The new standard requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The standard is effective for annual periods beginning after December 15, 2017, and interim periods within those annual periods. We believe that the adoption of this new standard will have no material impact on our financial position or results of operations and have not elected to early adopt the amendment. In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) Accounting Pronouncements Adopted In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting The impact of adopting ASU 2016-09 resulted in the following: • Classification of excess income tax benefits from stock-based compensation arrangement as a discrete item within income tax expense, rather than recognizing such excess income tax benefits in additional paid-in capital. The adoption of this guidance resulted in an increase of approximately $1.1 million of net operating losses, which has an impact of $0.4 million on our deferred tax assets before our full valuation allowance established against the related deferred tax assets. • Recognition of forfeitures as they occur. The cumulative effect adjustment as a result of the adoption of ASU 2016-09 on a modified retrospective basis was insignificant. • A change in classification of cash flows resulting from excess tax benefits and cash paid by us when directly withholding shares for tax-withholding purposes on a retrospective basis. The adoption of these provisions did not result in changes in our condensed consolidated statements of cash flow. There were no other material impacts to our condensed consolidated financial statements as a result of adopting this updated standard.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring Basis	The following table summarizes our financial assets measured at fair value on a recurring basis (in thousands): December 31, 2017 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 134,744 $ 134,744 $ — $ — Corporate debt securities 46,977 — 46,977 — U.S. government agency securities 54,989 — 54,989 — Certificates of deposits 9,129 — 9,129 — U.S. government securities 20,007 — 20,007 — Total financial assets $ 265,846 $ 134,744 $ 131,102 $ — December 31, 2016 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 72,597 $ 72,597 $ — $ — Corporate debt securities 48,531 — 48,531 — U.S. government agency securities 39,712 — 39,712 — Certificates of deposits 19,117 — 19,117 — U.S. government securities 7,999 — 7,999 — Total financial assets $ 187,956 $ 72,597 $ 115,359 $ —

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities	The following table is a summary of available-for-sale securities recorded in cash and cash equivalents, restricted cash, or marketable securities in our Consolidated Balance Sheets (in thousands): December 31, 2017 December 31, 2016 Amortized Unrealized Unrealized Estimated Fair Amortized Unrealized Unrealized Estimated Fair Financial Assets: Money market funds $ 134,744 $ — $ — $ 134,744 $ 72,597 $ — $ — $ 72,597 Corporate debt securities 47,108 — (131 ) 46,977 48,594 2 (65 ) 48,531 U.S. government agency securities 55,170 — (181 ) 54,989 39,763 1 (52 ) 39,712 Certificates of deposits 9,142 — (13 ) 9,129 19,169 — (52 ) 19,117 U.S. government securities 20,018 — (11 ) 20,007 7,999 — — 7,999 Total $ 266,182 $ — $ (336 ) $ 265,846 $ 188,122 $ 3 $ (169 ) $ 187,956
Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets	The following table summarizes the classification of the available-for-sale securities on our Consolidated Balance Sheets (in thousands): December 31, 2017 December 31, 2016 Cash and cash equivalents $ 134,746 $ 82,696 Short-term marketable securities 116,493 55,202 Long-term marketable securities 14,607 50,058 Total $ 265,846 $ 187,956

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2017 2016 Laboratory equipment $ 5,715 $ 3,895 Computer equipment 1,594 972 Leasehold improvements 12,642 678 Construction-in-progress 419 137 Total property and equipment 20,370 5,682 Less: accumulated depreciation and amortization (3,799 ) (3,262 ) Property and equipment, net $ 16,571 $ 2,420
Accrued Liabilities	Accrued liabilities consist of the following (in thousands): December 31, 2017 2016 Accrued clinical and manufacturing expenses $ 8,035 $ 4,759 Accrued professional and consulting services 1,007 507 Other 1,093 53 Total accrued liabilities $ 10,135 $ 5,319
Other Liabilities	Other liabilities consist of the following (in thousands): December 31, 2017 2016 Restricted shares subject to repurchase, current $ 373 $ 846 Deferred rent, current — 61 Other payable — 2 Total other liabilities, current $ 373 $ 909 Restricted shares subject to repurchase, noncurrent $ 161 $ 534 Deferred rent, noncurrent 11,491 29 Total other liabilities, noncurrent $ 11,652 $ 563

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance on Converted basis	We have reserved shares of common stock, on an as-converted basis, for future issuance as follows: December 31, 2017 2016 Restricted shares subject to future vesting 241,617 672,112 Restricted stock units 820,713 — Options issued and outstanding 2,945,901 2,769,702 Shares available for future grant under the 2015 Plan and 2017 Inducement Equity Plan 1,708,680 1,401,153 Employee stock purchase plan 186,033 76,118 Total 5,902,944 4,919,085
Schedule of Restricted Shares Subject to Repurchase	Restricted shares subject to repurchase were as follows: December 31, 2017 2016 Restricted shares subject to repurchase: Shares issued pursuant to the 2012 Stock Option and Grant Plan 241,617 672,112 Total restricted shares subject to repurchase 241,617 672,112

Share-based Compensation (Table

Share-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Stock Option Activity	The following table summarizes activity under the Company’s stock option plans, including the 2017 Inducement Plan, 2015 Plan and the 2012 Plan and related information (in thousands, except share and per share amounts and term): Number Weighted- Weighted- Aggregate Outstanding — December 31, 2016 2,769,702 $ 11.99 7.81 Options granted 1,001,990 25.08 Options exercised (578,455) 4.87 Options canceled (247,336) 16.12 Outstanding — December 31, 2017 2,945,901 $ 17.50 8.25 $ 65,041 Vested and exercisable — December 31, 2017 1,259,657 $ 13.91 7.74 $ 32,427 Vested and expected to vest — December 31, 2017 2,945,901 $ 17.50 8.25 $ 65,041
Valuation Assumptions for Stock Awards	The fair values of stock options granted to employees were calculated using the following assumptions: Year Ended December 31, 2017 2016 2015 Expected term (in years) 5.3-6.1 5.3-6.1 5.3-6.3 Volatility 68.9%-75.6% 70.6%-82.3% 73.8%-87.6% Risk-free interest rate 1.8%-2.3% 1.1%-2.1% 1.5%-1.8% Dividend yield — — —
Fair Value Assumptions for Non-Employee Awards	The fair value of the stock options granted during years ended December 31, 2016 and December 31, 2015 is calculated at each reporting date using the Black-Scholes option-pricing model with the following assumptions: Year Ended December 31, 2016 2015 Expected term (in years) 9.5 4.0-9.9 Volatility 78.0% 73.4%-91.2% Risk-free interest rate 1.74% 0.8-2.7% Dividend yield — —
Stock-based Compensation Expense Recognized	Total stock-based compensation recognized by our research and development function and our general and administrative function was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Research and development $ 5,905 $ 4,153 $ 2,031 General and administrative 7,777 5,082 1,192 Total stock-based compensation expense $ 13,682 $ 9,235 $ 3,223 Total stock-based compensation recognized by employees and non-employees was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Employee $ 13,682 $ 9,003 $ 2,359 Non-employee — 232 864 Total stock-based compensation expense $ 13,682 $ 9,235 $ 3,223
Unrecognized Stock-based Compensation Cost	As of December 31, 2017 the unrecognized stock-based compensation cost, net of expected forfeitures, and the estimated weighted-average amortization period, using the straight-line attribution method, was as follows (in thousands, except amortization period): Unrecognized Compensation Cost Weighted-average remaining amortization period (years) Options $ 20,969 2.4 Restricted stock purchases 300 0.9 Restricted stock units 13,670 2.2 ESPP 53 0.1 Total stock-based compensation expense $ 34,992 2.3
Restricted Stock Units [Member]
Non-Vested Restricted Stock Activity	The following table summarizes activity of RSUs granted to employees with service-based vesting under the 2017 Inducement Plan and 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested units — December 31, 2016 — $ — — $ — RSUs granted 520,940 24.74 RSUs vested (42,679 ) 21.48 RSUs forfeited (10,798 ) 29.24 Non-vested units — December 31, 2017 467,463 $ 24.93 1.71 $ 18,395
Market Condition Awards Granted to Employees [Member]
Summary of Activity of Market-Condition Awards	The following table summarizes activity of the market-condition awards under the 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested market-condition awards — December 31, 2016 — $ — — Granted 365,250 15.15 Vested — — Forfeited (12,000 ) 15.15 Non-vested market-condition awards — December 31, 2017 353,250 $ 15.15 0.73 $ 13,900
Valuation Assumptions of Market-Condition Awards	The following table summarizes the assumptions used to estimate the fair value of the market-condition awards during the year ended December 31, 2017: Valuation date stock price $ 28.55 Volatility 65.6 % Risk-free interest rate 1.4 % Dividend yield —
Restricted Stock Purchases [Member]
Non-Vested Restricted Stock Activity	A summary of our unvested restricted stock for the year ended December 31, 2017 is as follows: Number of RSPs Weighted Average Outstanding — December 31, 2016 672,112 $ 1.10 RSPs vested (306,389 ) 0.90 Repurchased by Company (124,106 ) 0.84 Outstanding — December 31, 2017 241,617 $ 1.46
ESPP [Member]
Fair Value Assumptions for Employee Stock Purchase Plan	The fair values of the rights granted under the 2015 ESPP were calculated using the following assumptions: Year Ended Year Ended Expected term (in years) 0.5 0.5 Volatility 60.1-63.5 % 83.1-84.6 % Risk-free interest rate 0.7-1.2 % 0.4-0.5 % Dividend yield — % — %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Components of Loss Before Income Taxes	The components of the loss before income taxes were as follows (in thousands): Year Ended December 31, 2017 2016 2015 Loss before provision for income taxes: United States $ (101,288 ) $ (70,103 ) $ (46,360) International (15,736 ) (12,365 ) — $ (117,024 ) $ (82,468 ) $ (46,360)
Effective Income Tax Rate Reconciliation	The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2017 2016 2015 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % Non-deductible changes in fair value and other 0.7 3.9 (4.8 ) Federal and state tax credits 7.3 8.3 1.4 Change in valuation allowance (14.8 ) (41.1 ) (30.6 ) Foreign rate differential (4.6 ) (5.1 ) — Officer compensation limitation (0.9 ) — — Tax reform – tax rate change (21.7 ) 0.0 % 0.0 % Provision for Taxes 0.0 % 0.0 % 0.0 %
Components of Deferred Tax Assets and Liabilities	The components of the deferred tax assets and liabilities are as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating loss carryforwards $ 51,448 $ 51,348 Tax credits 32,732 15,167 Property and equipment 116 9 Accruals and reserves 1,668 1,739 Stock based compensation 1,915 1,859 Gross deferred tax assets 87,879 70,122 Valuation allowance (87,879 ) (70,122 )
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2017 2016 Balance at beginning of year $ 5,296 $ 1,005 Additions based on tax positions related to current year 6,346 4,291 Decreased for prior period positions (492) Unrecognized tax benefit - December 31 $ 11,150 $ 5,296

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Future Aggregate Minimum Lease Payments	Future aggregate minimum lease payments under the noncancelable operating leases are as follows (in thousands): Year ending December 31, Amount 2018 3,265 2019 4,400 2020 4,545 2021 4,694 Thereafter 31,360 Total $ 48,264
Rent Expense	Rent expense for the facility operating leases consisted of the following (in thousands): Year Ended December 31, 2017 2016 2015 Minimum rental $ 2,001 $ 1,272 $ 971 Net reimbursement under Space Sharing Agreement — — (10 ) Facility rental expense, net $ 2,001 $ 1,272 $ 961

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Potentially Dilutive Securities that were Not Included in Diluted Net Loss per Share Calculations	The following securities were not included in the diluted net loss per share calculations because their effect was anti-dilutive: December 31, 2017 2016 2015 Options to purchase common stock 2,945,901 2,769,702 2,058,787 Restricted stock subject to future vesting 241,617 672,112 1,097,288 Restricted stock units 820,713 — — Common stock potentially issuable for ESPP purchases — 8,386 9,491 Total 4,008,231 3,450,200 3,165,566

Selected Quarterly Financial 29

Selected Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Consolidated Quarterly Financial Information (Unaudited)	The following table provides the selected consolidated quarterly financial data for 2017 and 2016: Quarter Ended (in thousands, except per share amounts) December 2017 September 30, 2017 June 30, 2017 March 31, 2017 December 31, 2016 September 30, 2016 June 30, 2016 March 31, 2016 Loss from operations $ (41,914 ) $ (29,180 ) $ (24,430 ) $ (23,721 ) $ (27,460 ) $ (21,144 ) $ (17,806 ) $ (16,717 ) Net loss $ (41,252 ) $ (28,557 ) $ (23,883 ) $ (23,332 ) $ (27,208 ) $ (20,985 ) $ (17,675 ) $ (16,600 ) Basic and diluted net loss per common share $ (0.94 ) $ (0.66 ) $ (0.55 ) $ (0.60 ) $ (0.74 ) $ (0.58 ) $ (0.58 ) $ (0.56 )

Organization and Basis of Pre30

Organization and Basis of Presentation - Additional Information (Detail)	1 Months Ended					12 Months Ended
	Jan. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Aug. 31, 2017USD ($)	Feb. 28, 2017USD ($)$ / sharesshares	Aug. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2017USD ($)Segment$ / sharesshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Class of Stock [Line Items]
Number of operating segments | Segment						1
Proceeds from issuance of common stock, net of issuance costs						$ 231,955,000	$ 116,995,000	$ 126,230,000
Accumulated deficit		$ (297,957,000)				$ (297,957,000)	$ (180,933,000)
Cowen and Company, LLC [Member] | Maximum [Member]
Class of Stock [Line Items]
Common stock sales offering price			$ 125,000,000
Common stock sales agreement commission percentage of gross proceeds per share sold			3.00%
Common Stock [Member]
Class of Stock [Line Items]
Common stock shares sold | shares		2,631,579		5,867,347		8,498,926	6,667,228	6,900,000
Share price per share | $ / shares		$ 38		$ 24.50		$ 38
Proceeds from IPO					$ 126,200,000
Proceeds from issuance of common stock, net of issuance costs		$ 96,400,000
Common Stock [Member] | Conversion of Redeemable Convertible Preferred Stock to Common Stock [Member]
Class of Stock [Line Items]
Shares converted upon completion of IPO (shares) | shares					19,746,614
Common Stock [Member] | Cowen and Company, LLC [Member]
Class of Stock [Line Items]
Common stock shares sold | shares						0
Common Stock [Member] | Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock shares sold | shares				765,306
Share price per share | $ / shares				$ 24.50
Proceeds from issuance of common stock, net of issuance costs				$ 135,600,000
Common Stock [Member] | Over-Allotment Option [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Common stock shares sold | shares	394,736
Share price per share | $ / shares	$ 38
Proceeds from issuance of common stock, net of issuance costs	$ 14,500,000
Common Stock [Member] | IPO [Member]
Class of Stock [Line Items]
Common stock shares sold | shares					6,900,000
Share price per share | $ / shares					$ 20

Summary of Significant Accoun31

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Impairment of long-lived assets	$ 0	$ 0	$ 0
Deferred tax assets	87,879,000	$ 70,122,000
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase in net operating loss carryforwards	1,100,000
Deferred tax assets	$ 400,000
Computer Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Laboratory Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	5 years

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Assets Measured on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 265,846	$ 187,956
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	20,007	7,999
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	134,744	72,597
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	131,102	115,359
Level 2 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	20,007	7,999
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	134,744	72,597
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	134,744	72,597
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	46,977	48,531
Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	46,977	48,531
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	54,989	39,712
U.S. Government Agency Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	54,989	39,712
Certificates of Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	9,129	19,117
Certificates of Deposits [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 9,129	$ 19,117

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair value assets amount transfer from level 1 to level 2	$ 0	$ 0
Fair value assets amount transfer from level 2 to level 1	$ 0	$ 0

Available-for-Sale Securities -

Available-for-Sale Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	$ 266,182	$ 188,122
Available for sale securities, Unrealized Gains		3
Available for sale securities, Unrealized (Losses)	(336)	(169)
Available for sale securities, Estimated Fair Value	265,846	187,956
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	20,018	7,999
Available for sale securities, Unrealized (Losses)	(11)
Available for sale securities, Estimated Fair Value	20,007	7,999
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	134,744	72,597
Available for sale securities, Estimated Fair Value	134,744	72,597
Certificates of Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	9,142	19,169
Available for sale securities, Unrealized (Losses)	(13)	(52)
Available for sale securities, Estimated Fair Value	9,129	19,117
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	47,108	48,594
Available for sale securities, Unrealized Gains		2
Available for sale securities, Unrealized (Losses)	(131)	(65)
Available for sale securities, Estimated Fair Value	46,977	48,531
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Amortized Cost	55,170	39,763
Available for sale securities, Unrealized Gains		1
Available for sale securities, Unrealized (Losses)	(181)	(52)
Available for sale securities, Estimated Fair Value	$ 54,989	$ 39,712

Available-for-Sale Securities35

Available-for-Sale Securities - Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 134,746	$ 82,696
Short-term marketable securities	116,493	55,202
Long-term marketable securities	14,607	50,058
Total	$ 265,846	$ 187,956

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 20,370	$ 5,682
Less: accumulated depreciation and amortization	(3,799)	(3,262)
Property and equipment, net	16,571	2,420
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	5,715	3,895
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	1,594	972
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	12,642	678
Construction-in-Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 419	$ 137

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance Sheet Components [Abstract]
Depreciation expense	$ 1.7	$ 1.2	$ 0.9

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Components [Abstract]
Accrued clinical and manufacturing expenses	$ 8,035	$ 4,759
Accrued professional and consulting services	1,007	507
Other	1,093	53
Total accrued liabilities	$ 10,135	$ 5,319

Balance Sheet Components - Othe

Balance Sheet Components - Other Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Components [Abstract]
Restricted shares subject to repurchase, current	$ 373	$ 846
Deferred rent, current		61
Other payable		2
Total other liabilities, current	373	909
Restricted shares subject to repurchase, noncurrent	161	534
Deferred rent, noncurrent	11,491	29
Total other liabilities, noncurrent	$ 11,652	$ 563

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)
Equity [Abstract]
Common stock dividend declared	$ 0

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Issuance on Converted Basis (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Shares reserved for future issuance	5,902,944	4,919,085
Restricted Stock [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	241,617	672,112
RSU [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	820,713
Stock Options [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	2,945,901	2,769,702
Shares Available for Future Grant Under the 2015 Plan and 2017 Inducement Equity Plan [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	1,708,680	1,401,153
ESPP [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	186,033	76,118

Stockholders' Equity - Schedu42

Stockholders' Equity - Schedule of Restricted Shares Subject to Repurchase (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Total restricted shares subject to repurchase	241,617	672,112
Restricted Stock [Member]
Class of Stock [Line Items]
Shares issued pursuant to the 2012 Stock Option and Grant Plan	241,617	672,112
2012 Stock Option and Grant Plan [Member] | Restricted Stock [Member]
Class of Stock [Line Items]
Shares issued pursuant to the 2012 Stock Option and Grant Plan	241,617	672,112

Share-based Compensation - Plan

Share-based Compensation - Plan Summary - Additional Information (Detail)	Aug. 11, 2017shares	Apr. 09, 2015USD ($)$ / sharesshares	Dec. 31, 2017USD ($)shares	Sep. 30, 2015USD ($)shares	Jul. 31, 2015shares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)Employee$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2012shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			5,902,944			5,902,944	4,919,085
Intrinsic value of options exercised | $						$ 11,300,000	$ 2,400,000	$ 400,000
Estimated weighted-average grant-date fair value of common stock underlying options granted | $ / shares						$ 16.19	$ 11.74	$ 8.56
Dividend yield						0.00%
Grants in period						1,001,990
Royalty fee as a percentage of future sales, less than				1.00%
Value of shares issued in research agreement | $								$ 4,492,000
Unrecognized compensation expense | $			$ 34,992,000			$ 34,992,000
Stock-based compensation | $						13,682,000	$ 9,235,000	3,223,000
Shares issued, value | $						1,050,000	1,018,000
Total stock-based compensation expense | $						$ 13,682,000	$ 9,235,000	$ 3,223,000
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued in research agreement				85,714				85,714
Value of shares issued in research agreement | $				$ 4,500,000
Shares issued						76,585	65,252
Non-employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period			0
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense | $						$ 5,905,000	$ 4,153,000	$ 2,031,000
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense | $						$ 7,777,000	$ 5,082,000	$ 1,192,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			2,945,901			2,945,901	2,769,702
Dividend yield						0.00%	0.00%	0.00%
weighted average derived service period						2 years 4 months 24 days
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			186,033			186,033	76,118
Dividend yield						0.00%	0.00%
weighted average derived service period						1 month 6 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			241,617			241,617	672,112
Restricted shares granted in period						0
weighted average derived service period						10 months 25 days
RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			820,713			820,713
weighted average derived service period						2 years 2 months 12 days
Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield						0.00%
Unrecognized compensation expense | $			$ 3,200,000			$ 3,200,000
weighted average derived service period						1 year 1 month 13 days
Stock-based compensation | $						$ 2,200,000
Market Condition Awards Granted to Employees [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period		24 months
Restricted shares granted in period		99,285
Minimum market capitalization | $		$ 2,000,000,000
Minimum consecutive trading days for market capitalization		20 days
Exercise price of restricted shares granted | $ / shares		$ 3.40
2017 Inducement Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			300,000			300,000
2017 Inducement Equity Plan [Member] | Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			170,000			170,000
2015 Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance					1,430,000
Expiration period						10 years
Exercise price as a percentage of the fair market value						100.00%
Exercise price as a percentage of the fair market value for option holding more than 10% total combined voting power						110.00%
Vesting period						4 years
2015 Plan [Member] | RSU [Member] | Senior Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted in period	365,250
2015 Plan [Member] | Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted in period						365,250
2012 Stock Option and Grant Plan [Member] | ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance		3,785,713							2,785,713
Expiration period						10 years
Increase in shares authorized for issuance under plan (shares)		1,000,000
2015 ESPP [Member] | ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price as a percentage of the fair market value					85.00%
Offering period					6 months
Maximum contribution to plan as a percent of employee's eligible compensation					15.00%
Shares issued						76,585
Shares issued, value | $						$ 1,000,000
Stock Option Modified [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense | $							$ 1,500,000
Number of employees affected | Employee							2
Stock Option Modified [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense | $							$ 900,000
Stock Option Modified [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense | $							$ 600,000

Share-based Compensation - Stoc

Share-based Compensation - Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, outstanding, Number of Options | shares	2,769,702
Options granted, Number of Options | shares	1,001,990
Options exercised, Number of Options | shares	(578,455)
Options canceled, Number of Options | shares	(247,336)
Ending balance, outstanding, Number of Options | shares	2,945,901	2,769,702
Vested and exercisable, Number of Options | shares	1,259,657
Vested and expected to vest, Number of Options | shares	2,945,901
Weighted- Average Exercise Price
Beginning balance, Outstanding, Weighted-Average Exercise Price | $ / shares	$ 11.99
Options granted, Weighted-Average Exercise Price | $ / shares	25.08
Options exercised, Weighted-Average Exercise Price | $ / shares	4.87
Options canceled, Weighted-Average Exercise Price | $ / shares	16.12
Ending balance, Outstanding, Weighted-Average Exercise Price | $ / shares	17.50	$ 11.99
Ending balance, Outstanding, Weighted-Average Exercise Price | $ / shares	13.91
Vested and exercisable, Weighted-Average Exercise Price | $ / shares	$ 17.50
Beginning, Outstanding, Weighted-Average remaining contractual term	8 years 2 months 30 days	7 years 9 months 22 days
Vested and exercisable, Weighted-Average remaining contractual term	7 years 8 months 26 days
Vested and expected to vest, Weighted-Average remaining contractual term	8 years 2 months 30 days
Balance Outstanding, Aggregate Intrinsic Value | $	$ 65,041
Vested and exercisable, Aggregate Intrinsic Value | $	32,427
Vested and expected to vest, Aggregate Intrinsic Value | $	$ 65,041

Share-based Compensation - Valu

Share-based Compensation - Valuation Assumptions for Stock Options Granted (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	68.90%	70.60%	73.80%
Volatility, maximum	75.60%	82.30%	87.60%
Risk-free interest rate, minimum	1.80%	1.10%	1.50%
Risk-free interest rate, maximum	2.30%	2.10%	1.80%
Dividend yield	0.00%	0.00%	0.00%
Stock Options [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 19 days	5 years 3 months 19 days	5 years 3 months 19 days
Stock Options [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 3 months 19 days

Share-based Compensation - Fair

Share-based Compensation - Fair Value Assumptions of Stock Option Awards and Performance-Contingent Awards (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	9 years 6 months
Volatility	78.00%
Risk-free interest rate	1.74%
Dividend yield	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		4 years
Volatility		73.40%
Risk-free interest rate		0.80%
Dividend yield		0.00%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		9 years 10 months 25 days
Volatility		91.20%
Risk-free interest rate		2.70%
Dividend yield		0.00%

Share-based Compensation - Summ

Share-based Compensation - Summary of RSU Activity Granted to Employees with Service-Based Vesting (Detail) - 2017 Inducement Plan and 2015 Plan [Member] - RSU [Member] $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs granted, Number of RSUs | shares	520,940
RSUs vested, Number of RSUs | shares	(42,679)
RSUs forfeited, Number of RSUs | shares	(10,798)
Ending Balance - Non-vested market-condition awards | shares	467,463
RSUs granted, Weighted-Average Grant Date Fair Value | $ / shares	$ 24.74
RSUs vested, Weighted-Average Grant Date Fair Value | $ / shares	21.48
RSUs forfeited, Weighted-Average Grant Date Fair Value | $ / shares	29.24
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 24.93
Non-vested units, Weighted-Average remaining vesting period(years)	1 year 8 months 16 days
Non-vested units, Aggregate Intrinsic Value | $	$ 18,395

Share-based Compensation - Unve

Share-based Compensation - Unvested Restricted Stock Activity (Detail) - Restricted Stock [Member]	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards | shares	672,112
Number of RSPs vested | shares	(306,389)
Number of RSPs Repurchased by Company | shares	(124,106)
Ending Balance - Non-vested market-condition awards | shares	241,617
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 1.10
Weighted Average Grant Date Fair Value per Share, RSPs vested | $ / shares	0.90
Weighted Average Grant Date Fair Value per Share, Repurchased by Company | $ / shares	0.84
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 1.46

Share-based Compensation - Perf

Share-based Compensation - Performance-Contingent Awards Granted to Employees - Additional Information (Detail)	Apr. 09, 2015Milestone$ / sharesshares	Sep. 30, 2017shares	Dec. 31, 2017$ / sharesshares	Dec. 31, 2016Milestoneshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period			1,001,990
Exercise price of options granted | $ / shares			$ 25.08
Senior Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of operating milestones for performance vesting | Milestone	4
Number of operating milestones achieved | Milestone				1
Performance Shares [Member] | Senior Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period	326,424
Performance Contingent Awards Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested				94,502
Number of shares forfeited		30,710		47,500
Performance Contingent Awards Stock Options [Member] | Senior Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period	227,139
Exercise price of options granted | $ / shares	$ 3.40
Performance Contingent Awards Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested				49,643
Number of shares repurchased		24,821		24,821
Performance Contingent Awards Restricted Stock [Member] | Senior Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted in period	99,285
Exercise price of restricted shares granted | $ / shares	$ 3.40
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted in period			0
Number of shares vested			306,389
Number of shares repurchased			124,106
Restricted Stock [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value of restricted shares | $ / shares	$ 0.70
Expected term	2 years 4 months 24 days

Share-based Compensation - Su50

Share-based Compensation - Summary of Activity of Market-Condition Awards (Detail) - Market Condition Awards Granted to Employees [Member] $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested units, Aggregate Intrinsic Value | $	$ 13,900
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of units granted | shares	365,250
Number of units vested | shares	0
Number of units forfeited | shares	(12,000)
Ending Balance - Non-vested market-condition awards | shares	353,250
Weighted-Average Grant Date Fair Value granted | $ / shares	$ 15.15
Weighted-Average Grant Date Fair Value vested | $ / shares	0
Weighted-Average Grant Date Fair Value forfeited | $ / shares	15.15
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 15.15
Non-vested units, Weighted-Average remaining vesting period(years)	8 months 23 days

Share-based Compensation - Va51

Share-based Compensation - Valuation Assumptions of Market-Condition Awards (Detail)	12 Months Ended
	Dec. 31, 2017$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%
Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Valuation date stock price	$ 28.55
Volatility	65.60%
Risk-free interest rate	1.40%
Dividend yield	0.00%

Share-based Compensation - Fa52

Share-based Compensation - Fair Value Assumptions for Employee Stock Purchase Plan (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months	6 months
Volatility	0.00%	0.00%
Volatility, minimum	60.10%	83.10%
Volatility, maximum	63.50%	84.60%
Risk-free interest rate	0.00%	0.00%
Risk-free interest rate, minimum	0.70%	0.40%
Risk-free interest rate, maximum	1.20%	0.50%
Dividend yield	0.00%	0.00%

Share-based Compensation - St53

Share-based Compensation - Stock-based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 13,682	$ 9,235	$ 3,223
Total stock-based compensation expense	13,682	9,235	3,223
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	5,905	4,153	2,031
Total stock-based compensation expense	5,905	4,153	2,031
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	7,777	5,082	1,192
Total stock-based compensation expense	7,777	5,082	1,192
Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	13,682	9,003	2,359
Total stock-based compensation expense	$ 13,682	9,003	2,359
Non-employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense		232	864
Total stock-based compensation expense		$ 232	$ 864

Share-based Compensation - Unre

Share-based Compensation - Unrecognized Stock-based Compensation Cost (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost	$ 34,992
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, options	$ 20,969
Weighted-average remaining amortization period	2 years 4 months 24 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, restricted stock awards	$ 300
Weighted-average remaining amortization period	10 months 25 days
RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, restricted stock awards	$ 13,670
Weighted-average remaining amortization period	2 years 2 months 12 days
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, restricted stock awards	$ 53
Weighted-average remaining amortization period	1 month 6 days

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Contribution expenses	$ 300,000	$ 200,000	$ 33,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
U.S. corporate income tax rate		34.00%	34.00%	34.00%
Provision for income taxes		$ 0	$ 0	$ 0
Increase in valuation allowance		$ 17,800,000	$ 34,000,000	$ 16,700,000
Minimum [Member]
Income Taxes [Line Items]
Operating loss carry forward, expiration year		2,032
Maximum [Member]
Income Taxes [Line Items]
Operating loss carry forward, expiration year		2,036
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
U.S. corporate income tax rate		35.00%
Scenario, Forecast [Member] | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
U.S. corporate income tax rate	21.00%
Federal [Member]
Income Taxes [Line Items]
Operating loss carryforwards		$ 194,500,000
Tax credit carryforwards		30,000,000
State [Member]
Income Taxes [Line Items]
Operating loss carryforwards		151,900,000
Tax credit carryforwards		$ 3,400,000

Income Taxes - Components of Lo

Income Taxes - Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss before provision for income taxes:
United States	$ (101,288)	$ (70,103)	$ (46,360)
International	(15,736)	(12,365)
Loss before provision for income taxes	$ (117,024)	$ (82,468)	$ (46,360)

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
Non-deductible changes in fair value and other	0.70%	3.90%	(4.80%)
Federal and state tax credits	7.30%	8.30%	1.40%
Change in valuation allowance	(14.80%)	(41.10%)	(30.60%)
Foreign rate differential	(4.60%)	(5.10%)
Officer compensation limitation	(0.90%)
Tax reform - tax rate change	(21.70%)	0.00%	0.00%
Provision for Taxes	0.00%	0.00%	0.00%

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 51,448	$ 51,348
Tax credits	32,732	15,167
Property and equipment	116	9
Accruals and reserves	1,668	1,739
Stock based compensation	1,915	1,859
Gross deferred tax assets	87,879	70,122
Valuation allowance	$ (87,879)	$ (70,122)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 5,296	$ 1,005
Additions based on tax positions related to current year	6,346	4,291
Decreased for prior period positions	(492)
Unrecognized tax benefit - December 31	$ 11,150	$ 5,296

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Board_member	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Related Party Transaction [Line Items]
Related party expenses			$ 65,000
Third Rock Ventures [Member] | Board Member that is a Partner in TRV [Member]
Related Party Transaction [Line Items]
Number of board members | Board_member	1
Third Rock Ventures [Member] | Management and Advisory Services [Member]
Related Party Transaction [Line Items]
Related party expenses	$ 0	$ 0	$ 65,000
Outstanding payable	$ 0	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Mar. 31, 2017USD ($)ft²	Dec. 31, 2017USD ($)ft²CompanyLease_Agreement	Dec. 31, 2015USD ($)
Commitment And Contingencies [Line Items]
Facility size | ft²	67,185	36,740
Initial term	10 years
Future minimum rental payments under the Lease term	$ 48,300,000
Lease commencement date	Dec. 15, 2017
Operating lease term	10 years
Operating lease term, optional extension	10 years
Operating leases latest expiration date		2018-04
Number of companies in Space Sharing Agreement | Company		2
Reimbursements from Space Sharing Agreement			$ 33,000
Liabilities for loss contingencies		$ 0
Former Facility [Member]
Commitment And Contingencies [Line Items]
Number of lease agreements terminated | Lease_Agreement		1
Incremental depreciation associated with acceleration of the useful life		$ 100,000
Lease agreement expiry date		Dec. 31, 2017
New Facility [Member]
Commitment And Contingencies [Line Items]
Leasehold improvements		$ 12,400,000
Lease Agreements [Member] | New Facility [Member]
Commitment And Contingencies [Line Items]
Tenant inducement		11,100,000
Capital Resources [Member] | New Facility [Member]
Commitment And Contingencies [Line Items]
Tenant inducement		$ 1,300,000
Standby Letters of Credit [Member]
Commitment And Contingencies [Line Items]
Restricted cash	$ 900,000

Commitments and Contingencies63

Commitments and Contingencies - Future Aggregate Minimum Lease Payments (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 3,265
2,019	4,400
2,020	4,545
2,021	4,694
Thereafter	31,360
Total	$ 48,264

Commitments and Contingencies64

Commitments and Contingencies - Rent Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases, Rent Expense, Net [Abstract]
Minimum rental	$ 2,001,000	$ 1,272,000	$ 971,000
Net reimbursement under Space Sharing Agreement			(10,000)
Facility rental expense, net	$ 2,001,000	$ 1,272,000	$ 961,000

Net Loss per Share - Potentiall

Net Loss per Share - Potentially Dilutive Securities that were Not Included in Diluted Net Loss per Share Calculations (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	4,008,231	3,450,200	3,165,566
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	2,945,901	2,769,702	2,058,787
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	241,617	672,112	1,097,288
RSU [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	820,713
ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share		8,386	9,491

Selected Quarterly Financial 66

Selected Quarterly Financial Information (Unaudited) - Selected Consolidated Quarterly Financial Information (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (41,914)	$ (29,180)	$ (24,430)	$ (23,721)	$ (27,460)	$ (21,144)	$ (17,806)	$ (16,717)	$ (119,245)	$ (83,127)	$ (46,393)
Net loss	$ (41,252)	$ (28,557)	$ (23,883)	$ (23,332)	$ (27,208)	$ (20,985)	$ (17,675)	$ (16,600)	$ (117,024)	$ (82,468)	$ (46,360)
Basic and diluted net loss per common share	$ (0.94)	$ (0.66)	$ (0.55)	$ (0.60)	$ (0.74)	$ (0.58)	$ (0.58)	$ (0.56)	$ (2.76)	$ (2.48)	$ (3.95)