Stock-Based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense	$ 2,441	$ 50	$ 4,272	$ 195
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense	164	37	225	141
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense	$ 2,277	$ 13	$ 4,047	$ 54