Summary of Significant Accounting Policies - Schedule of Estimated Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 3,579	$ 2,651	$ 11,101	$ 8,028
Research and development [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,419	1,005	4,400	2,608
General and administrative [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 2,160	$ 1,646	$ 6,701	$ 5,420