Organization, Basis of Presentation and Summary of Significant Accounting Policies - Condensed Consolidated Statement of Operations and Comprehensive Loss (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 23,562	$ 11,812	$ 57,615	$ 23,490
Net loss	$ (12,843)	$ (12,354)	$ (49,670)	$ (41,477)
Basic and diluted net loss per share	$ (0.13)	$ (0.15)	$ (0.52)	$ (0.55)
Collaboration Agreements [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 23,538	$ 11,759	$ 57,378	$ 23,042
Adjustments [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (4,461)		$ (8,495)
Basic and diluted net loss per share	$ (0.04)		$ (0.09)
Adjustments [Member] | Collaboration Agreements [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ (4,461)		$ (8,495)
Balances without adoption of Topic 606 [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (17,304)		$ (58,165)
Basic and diluted net loss per share	$ (0.17)		$ (0.61)
Balances without adoption of Topic 606 [Member] | Collaboration Agreements [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 19,077		$ 48,883