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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 21,416	$ 8,253	$ 34,053	$ 11,678
Net loss	$ (16,640)	$ (12,491)	$ (36,827)	$ (29,123)
Basic and diluted net loss per share	$ (0.17)	$ (0.17)	$ (0.40)	$ (0.41)
Collaboration Agreements [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 21,289	$ 7,977	$ 33,840	$ 11,283
Adjustments [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (2,271)		$ (4,047)
Basic and diluted net loss per share	$ (0.02)		$ (0.04)
Adjustments [Member] | Collaboration Agreements [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ (2,271)		$ (4,047)
Balances without adoption of Topic 606 [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss	$ (18,911)		$ (40,874)
Basic and diluted net loss per share	$ (0.19)		$ (0.44)
Balances without adoption of Topic 606 [Member] | Collaboration Agreements [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 19,018		$ 29,793