Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	$ 2,806	$ 2,729	$ 9,781	$ 8,062
Stock-based compensation expense for non-employee awards	44	(192)	160	(109)
Total stock-based compensation	2,850	2,537	9,941	7,953
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	1,663	1,517	5,185	4,453
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense for employee awards	$ 1,143	$ 1,212	$ 4,596	$ 3,609