Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for employee awards	$ 4,002	$ 2,958	$ 6,975	$ 5,332
Stock-based compensation expense for non-employee awards	153	72	116	84
Total stock-based compensation	4,155	3,030	7,091	5,416
Research and Development Expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for employee awards	1,841	1,651	3,522	2,935
General and Administrative Expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for employee awards	$ 2,161	$ 1,307	$ 3,453	$ 2,397