Global Blood Therapeutics (GBT) 10-Q 6 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 01, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
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 Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		52,232,448

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 194,891	$ 198,332
Short-term marketable securities	188,337	116,493
Prepaid expenses and other current assets	8,951	9,487
Total current assets	392,179	324,312
Property and equipment, net	16,749	16,571
Long-term marketable securities	98,869	14,607
Restricted cash	2,395	1,046
Other assets, noncurrent	219	184
Total assets	510,411	356,720
Current liabilities:
Accounts payable	4,960	7,177
Accrued liabilities	13,844	10,135
Accrued compensation	7,179	8,579
Other liabilities, current	944	373
Total current liabilities	26,927	26,264
Other liabilities, noncurrent	11,283	11,652
Total liabilities	38,210	37,916
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized as of September 30, 2018 and December 31, 2017; no shares issued and outstanding
Common stock, $0.001 par value, 150,000,000 shares authorized as of September 30, 2018 (unaudited) and December 31, 2017, respectively; 52,120,387 and 46,131,723 shares issued and outstanding as of September 30, 2018 (unaudited) and December 31, 2017, respectively	52	46
Additional paid-in capital	895,478	617,051
Accumulated other comprehensive loss	(381)	(336)
Accumulated deficit	(422,948)	(297,957)
Total stockholders' equity	472,201	318,804
Total liabilities and stockholders' equity	$ 510,411	$ 356,720

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
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	52,120,387	46,131,723
Common stock, shares outstanding	52,120,387	46,131,723

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 33,026	$ 20,952	$ 94,543	$ 56,513
General and administrative	12,450	8,228	36,115	20,817
Total operating expenses	45,476	29,180	130,658	77,330
Loss from operations	(45,476)	(29,180)	(130,658)	(77,330)
Other income (expense):
Interest income, net	2,480	727	5,768	1,856
Other expenses, net	(72)	(104)	(101)	(298)
Total other income, net	2,408	623	5,667	1,558
Net loss	(43,068)	(28,557)	(124,991)	(75,772)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities, net of tax	(139)	71	(45)	(31)
Comprehensive loss	$ (43,207)	$ (28,486)	$ (125,036)	$ (75,803)
Basic and diluted net loss per common share	$ (0.83)	$ (0.66)	$ (2.47)	$ (1.81)
Weighted-average number of shares used in computing basic and diluted net loss per common share	52,050,232	43,259,145	50,536,860	41,832,273

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (124,991)	$ (75,772)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,509	1,008
Amortization (accretion) of premium (discount) on marketable securities	(226)	561
Stock-based compensation	22,552	8,941
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,496)	(2,873)
Accounts payable	(2,131)	(939)
Accrued liabilities	5,882	900
Accrued compensation	(1,400)	554
Other liabilities	499	(45)
Net cash used in operating activities	(101,802)	(67,665)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(4,946)	(1,773)
Purchase of marketable securities	(259,631)	(127,721)
Maturities of marketable securities	108,707	52,109
Net cash used in investing activities	(155,870)	(77,385)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock in public offering, net	255,119	135,625
Proceeds from issuance of common stock in settlement of employee stock purchase plan and exercise of stock options	6,722	3,066
Repurchases of unvested restricted stock	(8)	(421)
Tax paid related to net shares settlement of equity awards	(6,253)	(237)
Net cash provided by financing activities	255,580	138,033
Net decrease in cash, cash equivalents and restricted cash	(2,092)	(7,017)
Cash, cash equivalents and restricted cash at beginning of period	199,378	92,212
Cash, cash equivalents and restricted cash at end of period	197,286	85,195
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Accrued purchase of property and equipment	$ (2,173)	98
Leasehold improvements paid for by landlord		$ 9,885

Organization and Basis of Prese

Organization and Basis of Presentation	9 Months Ended
	Sep. 30, 2018
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. Follow-on In December 2017, we completed a follow-on In March 2018, we completed a follow-on 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. Since inception through September 30, 2018, we have incurred cumulative net losses of $422.9 million. We expect to incur additional losses in the future to conduct product research and development and we recognize the need to raise additional capital to fully implement our business plan. We intend to raise such capital through the issuance of additional equity, and potentially through borrowings, and strategic alliances with partner companies. However, if such financing is not available at adequate levels, we will need to re-evaluate

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Preparation The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These unaudited interim condensed consolidated financial statements have been prepared on the same basis as our annual consolidated financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of our financial information. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any other future year. The accompanying unaudited interim condensed consolidated financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2017 included in our Annual Report on Form 10-K, Use of Estimates The preparation of the accompanying unaudited interim condensed 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 condensed 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 unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated upon consolidation. Significant Accounting Policies There have been no material revisions in our significant accounting policies described in Note 2 to the consolidated financial statements included in our Annual Report on Form 10-K Accounting Pronouncements Adopted In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments No. 2016-15 In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash beginning-of-period end-of-period No. 2016-18 In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) No. 2017-09 In March 2018, the FASB issued ASU No. 2018-04, Investments – Debt Securities (Topic 320) and Regulated Operations (Topic 980): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 117 and SEC Release No. 33-9273. No. 2018-04 Investments-Debt Securities Regulated Operations In March 2018, the FASB issued ASU No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 No. 2018-05 Income Taxes In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting non-employee No. 2018-07 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) 2016-02). right-of-use No. 2018-01, Land Easement Practical Expedient for Transition to Topic 842 No. 2018-10, Codification Improvements to Topic 842, Leases No. 2018-11, Leases (Topic 842) – Targeted Improvements 2016-02, right-of-use In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income No. 2018-02 In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) No. 2018-13 No. 2018-13. In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal-Use 350-40), 2018-15). 2018-15 internal-use internal-use 2018-15. No. 2018-15 350-40 2018-15 2018-15

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
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 prepaid expenses and other current assets, 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 condensed consolidated balance sheets. The remaining financial instruments are reported on our condensed 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 condensed 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): September 30, 2018 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 187,825 $ 187,825 $ — $ — Corporate debt securities 81,082 — 81,082 — U.S. government agency securities 88,368 — 88,368 — Certificates of deposits 5,215 — 5,215 — U.S. government securities 114,259 — 114,259 — Total financial assets $ 476,749 $ 187,825 $ 288,924 $ — 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 $ — 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 A-1/P-1/F1

Available-for-Sale Securities

Available-for-Sale Securities	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Securities	4. Available-for-Sale Estimated fair values of available-for-sale available-for-sale September 30, 2018 December 31, 2017 Amortized Unrealized Unrealized Estimated Fair Amortized Unrealized Unrealized Estimated Fair Financial Assets: Money market funds $ 187,825 $ — $ — $ 187,825 $ 134,744 $ — $ — $ 134,744 Corporate debt securities 81,213 — (131 ) 81,082 47,108 — (131 ) 46,977 U.S. government agency securities 88,495 — (127 ) 88,368 55,170 — (181 ) 54,989 Certificates of deposits 5,216 1 (2 ) 5,215 9,142 — (13 ) 9,129 U.S. government securities 114,381 — (122 ) 114,259 20,018 — (11 ) 20,007 Total $ 477,130 $ 1 $ (382 ) $ 476,749 $ 266,182 $ — $ (336 ) $ 265,846 The following table summarizes the classification of the available-for-sale September 30, 2018 December 31, 2017 Cash and cash equivalents $ 189,543 $ 134,746 Short-term marketable securities 188,337 116,493 Long-term marketable securities 98,869 14,607 Total $ 476,749 $ 265,846 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 were temporary in nature during the periods presented.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and Equipment Property and equipment consists of the following (in thousands): September 30, 2018 December 31, 2017 Laboratory equipment $ 6,766 $ 5,715 Computer equipment 1,630 1,594 Leasehold improvements 13,730 12,642 Construction-in-progress 932 419 Total property and equipment 23,058 20,370 Less: accumulated depreciation and (6,309 ) (3,799 ) Property and equipment, net $ 16,749 $ 16,571 Accrued liabilities Accrued liabilities consist of the following (in thousands): September 30, 2018 December 31, 2017 Accrued clinical and manufacturing expenses $ 13,024 $ 8,035 Accrued professional and consulting services 652 1,007 Other 168 1,093 Total accrued liabilities $ 13,844 $ 10,135 Other liabilities, current and noncurrent Other liabilities consist of the following (in thousands): September 30, 2018 December 31, 2017 Restricted shares subject to repurchase, current $ 237 $ 373 Deferred rent, current 677 — Other payable 30 — Total other liabilities, current $ 944 $ 373 Restricted shares subject to repurchase, noncurrent $ — $ 161 Deferred rent, noncurrent 11,223 11,491 Other liabilities, noncurrent 60 — Total other liabilities, noncurrent $ 11,283 $ 11,652

Stock Based Compensation

Stock Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Based Compensation	6. Stock Based Compensation We have three stock-based compensation plans – the Amended and Restated 2017 Inducement Equity Plan (the “2017 Inducement Plan”), the 2015 Stock Option and Incentive Plan (the “2015 Plan”) and the 2012 Stock Option and Grant Plan (the “2012 Plan”). As of September 30, 2018, there were 646,600 shares reserved under the 2017 Inducement Plan and 2,543,178 shares reserved under the 2015 Plan for the future issuance of equity awards. Upon adoption of the 2015 Plan in July 2015, no new awards or grants are permitted under the 2012 Plan. See Note 7 to the Consolidated Financial Statements included in our Annual Report on Form 10-K Stock Options The following summarizes option activity under the 2017 Inducement Plan, 2015 Plan and 2012 Plan: Number of Weighted- Outstanding — December 31, 2017 2,945,901 $ 17.50 Options granted 928,958 55.74 Options exercised (527,278 ) 9.63 Options canceled (147,052 ) 33.74 Outstanding — September 30, 2018 3,200,529 $ 29.15 The fair values of stock options granted to employees were calculated using the following assumptions: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Expected term (in years) 6.1 6.0-6.1 5.3-6.1 5.3-6.1 Volatility 69.7%-70.3% 72.0%-73.1% 68.7%-70.8% 70.9%-75.6% Risk-free interest rate 2.8%-2.9% 1.9%-2.1% 2.6%-2.9% 1.8%-2.3% Dividend yield — — — — Restricted Stock Units 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: Number Weighted- Non-vested 467,463 $ 24.93 RSUs granted 529,985 57.68 RSUs vested (162,301 ) 34.36 RSUs forfeited (68,153 ) 41.12 Non-vested 766,994 $ 43.98 Market-Condition Awards Granted to Employees On August 11, 2017, our Board of Directors approved awards of 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. During the nine-month period ended September 30, 2018, certain market capitalization milestones were achieved, resulting in vesting of related market-condition RSUs. For the nine-month period ended September 30, 2018, we recognized $2.9 million in stock-based compensation expense related to the market-condition awards (allocated as $0.2 million for research and development expense and $2.7 million for general and administrative expense). The following table summarizes activity of the market-condition awards under the 2015 Plan and related information: Number Weighted- Non-vested 353,250 $ 15.15 Granted — — Vested (188,400 ) 18.22 Forfeited (5,600 ) 11.64 Non-vested 159,250 $ 11.64 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. As approved by the Compensation Committee of the Board of Directors in December 2017, the 2015 ESPP provides for offering periods of two years in duration with purchase periods occurring every six months during an offering period. 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. The fair values of the rights granted under the 2015 ESPP were calculated using the following assumptions: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Expected term (in years) 0.5 – 2.0 0.5 0.5 – 2.0 0.5 Volatility 59.2%-65.4% 60.1%-63.5% 59.2%-66.8% 60.1%-63.5% Risk-free interest rate 1.6%-2.7% 0.7%-1.2% 1.6%-2.7% 0.7%-1.2% Dividend yield — — — — Stock-Based Compensation Expense Total stock-based compensation recognized by function was as follows (in thousands): Three Months Ended Nine Months Ended 2018 2017 2018 2017 Research and development $ 2,835 $ 1,564 $ 9,587 $ 4,077 General and administrative 4,133 2,143 12,965 4,864 Total stock-based compensation expense $ 6,968 $ 3,707 $ 22,552 $ 8,941

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share	7. 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: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Options to purchase common stock 3,200,529 2,889,431 3,200,529 2,889,431 Restricted stock subject to future vesting 71,246 315,542 71,246 315,542 Restricted stock units 926,244 769,783 926,244 769,783 Total 4,198,019 3,974,756 4,198,019 3,974,756

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. 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 “Existing Premises”). 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 10-year In August 2018, we entered into an amendment to the Lease (the “Lease Amendment”) to relocate the leased premises from the Existing Premises to a to-be-constructed-building 10-year 10-year In March 2017, we provided a standby letter of credit of $0.9 million as security for our obligations under the Lease on our Existing Premises. The security deposit was increased to $2.4 million under the Lease Amendment. This standby letter of credit is classified as restricted cash. We intend to vacate the Existing Premises and surrender and deliver the Existing Premises to landlord on or before the date which is sixty days after the Substitute Premises Commencement Date, upon which time we will have no further obligations with respect to the Existing Premises. Upon signing of the Lease Amendment, we re-evaluated Future annual minimum lease payments due under the Lease and Lease Amendment at December 31 of each year are as follows (in thousands): Year ending December 31, Amount 1 2018 (three months) $ 1,076 2019 4,406 2020 6,513 2021 11,642 2022 12,020 Thereafter 102,776 Total $ 138,433 (1) The table above is prepared under the assumption that the Substitute Premises Commencement at the Substitute Premises starts on June 30, 2020. Rent expense for the three months ended September 30, 2018 and 2017 was $0.9 million and $0.4 million, respectively, and for the nine months ended September 30, 2018 and 2017 was $2.7 million and $1.1 million, respectively. The operating leases require us to share in prorated operating expenses and property taxes based upon actual amounts incurred; those amounts are not fixed for future periods and, therefore, are not included in the future commitments listed above. Contingencies In the ordinary course of business, we may be subject to legal claims and regulatory actions that could have a material adverse effect on our business or financial position. We assess our potential liability in such situations by analyzing potential outcomes, assuming various litigation, regulatory and settlement strategies. If we determine a loss is probable and its amount can be reasonably estimated, we accrue an amount equal to the estimated loss. No losses and no provision for a loss contingency have been recorded to date. Contingent Payments In August 2018, we entered into a license agreement (the “License Agreement”) with F. Hoffmann-La Hoffmann-La know-how non-exclusive,

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Preparation	Basis of Preparation The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2017 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These unaudited interim condensed consolidated financial statements have been prepared on the same basis as our annual consolidated financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of our financial information. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any other future year. The accompanying unaudited interim condensed consolidated financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2017 included in our Annual Report on Form 10-K,
Use of Estimates	Use of Estimates The preparation of the accompanying unaudited interim condensed 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 condensed 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 unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated upon consolidation.
Significant Accounting Policies	Significant Accounting Policies There have been no material revisions in our significant accounting policies described in Note 2 to the consolidated financial statements included in our Annual Report on Form 10-K
Accounting Pronouncements Adopted/Recent Accounting Pronouncements	Accounting Pronouncements Adopted In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments No. 2016-15 In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash beginning-of-period end-of-period No. 2016-18 In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) No. 2017-09 In March 2018, the FASB issued ASU No. 2018-04, Investments – Debt Securities (Topic 320) and Regulated Operations (Topic 980): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 117 and SEC Release No. 33-9273. No. 2018-04 Investments-Debt Securities Regulated Operations In March 2018, the FASB issued ASU No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 No. 2018-05 Income Taxes In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting non-employee No. 2018-07 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) 2016-02). right-of-use No. 2018-01, Land Easement Practical Expedient for Transition to Topic 842 No. 2018-10, Codification Improvements to Topic 842, Leases No. 2018-11, Leases (Topic 842) – Targeted Improvements 2016-02, right-of-use In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income No. 2018-02 In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) No. 2018-13 No. 2018-13. In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal-Use 350-40), 2018-15). 2018-15 internal-use internal-use 2018-15. No. 2018-15 350-40 2018-15 2018-15

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
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): September 30, 2018 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 187,825 $ 187,825 $ — $ — Corporate debt securities 81,082 — 81,082 — U.S. government agency securities 88,368 — 88,368 — Certificates of deposits 5,215 — 5,215 — U.S. government securities 114,259 — 114,259 — Total financial assets $ 476,749 $ 187,825 $ 288,924 $ — 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 $ —

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities	The following table is a summary of available-for-sale September 30, 2018 December 31, 2017 Amortized Unrealized Unrealized Estimated Fair Amortized Unrealized Unrealized Estimated Fair Financial Assets: Money market funds $ 187,825 $ — $ — $ 187,825 $ 134,744 $ — $ — $ 134,744 Corporate debt securities 81,213 — (131 ) 81,082 47,108 — (131 ) 46,977 U.S. government agency securities 88,495 — (127 ) 88,368 55,170 — (181 ) 54,989 Certificates of deposits 5,216 1 (2 ) 5,215 9,142 — (13 ) 9,129 U.S. government securities 114,381 — (122 ) 114,259 20,018 — (11 ) 20,007 Total $ 477,130 $ 1 $ (382 ) $ 476,749 $ 266,182 $ — $ (336 ) $ 265,846
Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets	The following table summarizes the classification of the available-for-sale September 30, 2018 December 31, 2017 Cash and cash equivalents $ 189,543 $ 134,746 Short-term marketable securities 188,337 116,493 Long-term marketable securities 98,869 14,607 Total $ 476,749 $ 265,846

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Property and Equipment	Property and equipment consists of the following (in thousands): September 30, 2018 December 31, 2017 Laboratory equipment $ 6,766 $ 5,715 Computer equipment 1,630 1,594 Leasehold improvements 13,730 12,642 Construction-in-progress 932 419 Total property and equipment 23,058 20,370 Less: accumulated depreciation and (6,309 ) (3,799 ) Property and equipment, net $ 16,749 $ 16,571
Accrued Liabilities	Accrued liabilities consist of the following (in thousands): September 30, 2018 December 31, 2017 Accrued clinical and manufacturing expenses $ 13,024 $ 8,035 Accrued professional and consulting services 652 1,007 Other 168 1,093 Total accrued liabilities $ 13,844 $ 10,135
Other Liabilities	Other liabilities consist of the following (in thousands): September 30, 2018 December 31, 2017 Restricted shares subject to repurchase, current $ 237 $ 373 Deferred rent, current 677 — Other payable 30 — Total other liabilities, current $ 944 $ 373 Restricted shares subject to repurchase, noncurrent $ — $ 161 Deferred rent, noncurrent 11,223 11,491 Other liabilities, noncurrent 60 — Total other liabilities, noncurrent $ 11,283 $ 11,652

Stock Based Compensation (Table

Stock Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Stock Option Activity	The following summarizes option activity under the 2017 Inducement Plan, 2015 Plan and 2012 Plan: Number of Weighted- Outstanding — December 31, 2017 2,945,901 $ 17.50 Options granted 928,958 55.74 Options exercised (527,278 ) 9.63 Options canceled (147,052 ) 33.74 Outstanding — September 30, 2018 3,200,529 $ 29.15
Valuation Assumptions for Stock Awards	The fair values of stock options granted to employees were calculated using the following assumptions: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Expected term (in years) 6.1 6.0-6.1 5.3-6.1 5.3-6.1 Volatility 69.7%-70.3% 72.0%-73.1% 68.7%-70.8% 70.9%-75.6% Risk-free interest rate 2.8%-2.9% 1.9%-2.1% 2.6%-2.9% 1.8%-2.3% Dividend yield — — — —
Stock-based Compensation Expense Recognized	Total stock-based compensation recognized by function was as follows (in thousands): Three Months Ended Nine Months Ended 2018 2017 2018 2017 Research and development $ 2,835 $ 1,564 $ 9,587 $ 4,077 General and administrative 4,133 2,143 12,965 4,864 Total stock-based compensation expense $ 6,968 $ 3,707 $ 22,552 $ 8,941
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: Number Weighted- Non-vested 467,463 $ 24.93 RSUs granted 529,985 57.68 RSUs vested (162,301 ) 34.36 RSUs forfeited (68,153 ) 41.12 Non-vested 766,994 $ 43.98
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: Number Weighted- Non-vested 353,250 $ 15.15 Granted — — Vested (188,400 ) 18.22 Forfeited (5,600 ) 11.64 Non-vested 159,250 $ 11.64
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: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Expected term (in years) 0.5 – 2.0 0.5 0.5 – 2.0 0.5 Volatility 59.2%-65.4% 60.1%-63.5% 59.2%-66.8% 60.1%-63.5% Risk-free interest rate 1.6%-2.7% 0.7%-1.2% 1.6%-2.7% 0.7%-1.2% Dividend yield — — — —

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2018
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: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Options to purchase common stock 3,200,529 2,889,431 3,200,529 2,889,431 Restricted stock subject to future vesting 71,246 315,542 71,246 315,542 Restricted stock units 926,244 769,783 926,244 769,783 Total 4,198,019 3,974,756 4,198,019 3,974,756

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Future Aggregate Minimum Lease Payments	Future annual minimum lease payments due under the Lease and Lease Amendment at December 31 of each year are as follows (in thousands): Year ending December 31, Amount 1 2018 (three months) $ 1,076 2019 4,406 2020 6,513 2021 11,642 2022 12,020 Thereafter 102,776 Total $ 138,433 (1) The table above is prepared under the assumption that the Substitute Premises Commencement at the Substitute Premises starts on June 30, 2020.

Organization and Basis of Pre_2

Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended			9 Months Ended
	Mar. 31, 2018USD ($)$ / sharesshares	Jan. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Sep. 30, 2018USD ($)Segment	Sep. 30, 2017USD ($)
Class of Stock [Line Items]
Number of operating segments | Segment				1
Proceeds from issuance of common stock, net of issuance costs				$ 255,119	$ 135,625
Cumulative net losses			$ (297,957)	$ (422,948)
Common Stock [Member]
Class of Stock [Line Items]
Common stock shares sold | shares	4,600,000		2,631,579
Share price per share | $ / shares	$ 54		$ 38
Proceeds from issuance of common stock, net of issuance costs			$ 96,400
Common Stock [Member] | Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock shares sold | shares	600,000	394,736
Share price per share | $ / shares	$ 54	$ 38
Proceeds from issuance of common stock, net of issuance costs	$ 240,600	$ 14,600

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Assets Measured on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 476,749	$ 265,846
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	114,259	20,007
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	187,825	134,744
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	288,924	131,102
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	114,259	20,007
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	187,825	134,744
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	187,825	134,744
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	81,082	46,977
Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	81,082	46,977
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	88,368	54,989
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	88,368	54,989
Certificates of Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	5,215	9,129
Certificates of Deposits [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 5,215	$ 9,129

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
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	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	$ 477,130	$ 266,182
Available for sale securities, Unrealized Gains	1
Available for sale securities, Unrealized (Losses)	(382)	(336)
Available for sale securities, Estimated Fair Value	476,749	265,846
U.S. Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	114,381	20,018
Available for sale securities, Unrealized (Losses)	(122)	(11)
Available for sale securities, Estimated Fair Value	114,259	20,007
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	187,825	134,744
Available for sale securities, Estimated Fair Value	187,825	134,744
Certificates of Deposits [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	5,216	9,142
Available for sale securities, Unrealized Gains	1
Available for sale securities, Unrealized (Losses)	(2)	(13)
Available for sale securities, Estimated Fair Value	5,215	9,129
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	81,213	47,108
Available for sale securities, Unrealized (Losses)	(131)	(131)
Available for sale securities, Estimated Fair Value	81,082	46,977
U.S. Government Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	88,495	55,170
Available for sale securities, Unrealized (Losses)	(127)	(181)
Available for sale securities, Estimated Fair Value	$ 88,368	$ 54,989

Available-for-Sale Securities_2

Available-for-Sale Securities - Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 189,543	$ 134,746
Short-term marketable securities	188,337	116,493
Long-term marketable securities	98,869	14,607
Total	$ 476,749	$ 265,846

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 23,058	$ 20,370
Less: accumulated depreciation and amortization	(6,309)	(3,799)
Property and equipment, net	16,749	16,571
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	6,766	5,715
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	1,630	1,594
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	13,730	12,642
Construction-in-Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 932	$ 419

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Balance Sheet Components [Abstract]
Accrued clinical and manufacturing expenses	$ 13,024	$ 8,035
Accrued professional and consulting services	652	1,007
Other	168	1,093
Total accrued liabilities	$ 13,844	$ 10,135

Balance Sheet Components - Othe

Balance Sheet Components - Other Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Balance Sheet Components [Abstract]
Restricted shares subject to repurchase,current	$ 237	$ 373
Deferred rent, current	677
Other payable	30
Total other liabilities, current	944	373
Restricted shares subject to repurchase, noncurrent		161
Deferred rent, noncurrent	11,223	11,491
Other liabilities, noncurrent	60
Total other liabilities, noncurrent	$ 11,283	$ 11,652

Stock Based Compensation - Plan

Stock Based Compensation - Plan Summary - Additional Information (Detail) - USD ($) $ in Thousands	Aug. 11, 2017	Jul. 31, 2015	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 22,552	$ 8,941
Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			2,900
Market Condition Awards Granted to Employees [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			200
Market Condition Awards Granted to Employees [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 2,700
2017 Inducement Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			646,600
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance			2,543,178
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			0
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		2 years
Maximum contribution to plan as a percent of employee's eligible compensation		15.00%

Stock Based Compensation - Stoc

Stock Based Compensation - Stock Option Activity (Detail)	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, outstanding, Number of Options | shares	2,945,901
Options granted, Number of Options | shares	928,958
Options exercised, Number of Options | shares	(527,278)
Options canceled, Number of Options | shares	(147,052)
Ending balance, outstanding, Number of Options | shares	3,200,529
Weighted- Average Exercise Price
Beginning balance, Outstanding, Weighted-Average Exercise Price | $ / shares	$ 17.50
Options granted, Weighted-Average Exercise Price | $ / shares	55.74
Options exercised, Weighted-Average Exercise Price | $ / shares	9.63
Options canceled, Weighted-Average Exercise Price | $ / shares	33.74
Ending balance, Outstanding, Weighted-Average Exercise Price | $ / shares	$ 29.15

Stock Based Compensation - Valu

Stock Based Compensation - Valuation Assumptions for Stock Options Granted (Detail) - Stock Options [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days
Volatility, minimum	69.70%	72.00%	68.70%	70.90%
Volatility, maximum	70.30%	73.10%	70.80%	75.60%
Risk-free interest rate, minimum	2.80%	1.90%	2.60%	1.80%
Risk-free interest rate, maximum	2.90%	2.10%	2.90%	2.30%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 years	5 years 3 months 18 days	5 years 3 months 18 days
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 1 month 6 days

Stock Based Compensation - Summ

Stock Based Compensation - Summary of RSU Activity Granted to Employees with Service-Based Vesting (Detail) - 2017 Inducement Plan and 2015 Plan [Member] - RSU [Member]	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards | shares	467,463
RSUs granted, Number of RSUs | shares	529,985
RSUs vested, Number of RSUs | shares	(162,301)
RSUs forfeited, Number of RSUs | shares	(68,153)
Ending Balance - Non-vested market-condition awards | shares	766,994
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 24.93
RSUs granted, Weighted-Average Grant Date Fair Value | $ / shares	57.68
RSUs vested, Weighted-Average Grant Date Fair Value | $ / shares	34.36
RSUs forfeited, Weighted-Average Grant Date Fair Value | $ / shares	41.12
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 43.98

Stock Based Compensation - Su_2

Stock Based Compensation - Summary of Activity of Market-Condition Awards (Detail) - 2015 Plan [Member] - Market Condition Awards Granted to Employees [Member]	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards | shares	353,250
Number of units granted | shares	0
Number of units vested | shares	(188,400)
Number of units forfeited | shares	(5,600)
Ending Balance - Non-vested market-condition awards | shares	159,250
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 15.15
Weighted-Average Grant Date Fair Value granted | $ / shares	0
Weighted-Average Grant Date Fair Value vested | $ / shares	18.22
Weighted-Average Grant Date Fair Value forfeited | $ / shares	11.64
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 11.64

Stock Based Compensation - Fair

Stock Based Compensation - Fair Value Assumptions for Employee Stock Purchase Plan (Detail) - ESPP [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 months		6 months
Volatility, minimum	59.20%	60.10%	59.20%	60.10%
Volatility, maximum	65.40%	63.50%	66.80%	63.50%
Risk-free interest rate, minimum	1.60%	0.70%	1.60%	0.70%
Risk-free interest rate, maximum	2.70%	1.20%	2.70%	1.20%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months		6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	2 years		2 years

Stock Based Compensation - St_2

Stock Based Compensation - Stock-based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 6,968	$ 3,707	$ 22,552	$ 8,941
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,835	1,564	9,587	4,077
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,133	$ 2,143	$ 12,965	$ 4,864

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	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	4,198,019	3,974,756	4,198,019	3,974,756
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	3,200,529	2,889,431	3,200,529	2,889,431
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	71,246	315,542	71,246	315,542
RSU [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	926,244	769,783	926,244	769,783

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Sep. 30, 2018USD ($)	Aug. 31, 2018USD ($)ft²	Mar. 31, 2017USD ($)ft²	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)
Commitment And Contingencies [Line Items]
Facility size | ft²		164,150	67,185
Initial term		10 years	10 years
Future minimum rental payments under the Lease term		$ 121,500,000	$ 48,300,000
Lease commencement date			Dec. 15, 2017
Operating lease term		10 years	10 years
Operating lease term, optional extension		10 years	10 years
Repayment of tenant inducement allowance		$ 4,100,000
Security deposit			$ 2,400,000
Rent expense				$ 900,000	$ 400,000	$ 2,700,000	$ 1,100,000
Contingency losses						0
Provisions for loss contingency						0
F. Hoffmann-La Roche Ltd. And Hoffmann-La Roche Inc. [Member]
Commitment And Contingencies [Line Items]
Upfront payments	$ 2,000,000
Contingent payments	125,500,000			$ 125,500,000		125,500,000
Royalty payments	$ 1,000,000,000
Standby Letters of Credit [Member]
Commitment And Contingencies [Line Items]
Restricted cash			$ 900,000
Lease Agreements [Member]
Commitment And Contingencies [Line Items]
Tenant inducement						11,100,000
Repayment of tenant inducement allowance						$ 1,700,000
Lease Agreements [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Tenant inducement		$ 27,900,000

Commitments and Contingencies_2

Commitments and Contingencies - Future Aggregate Minimum Lease Payments (Detail) $ in Thousands	Sep. 30, 2018USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018 (three months)	$ 1,076
2,019	4,406
2,020	6,513
2,021	11,642
2,022	12,020
Thereafter	102,776
Total	$ 138,433