Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for employee awards	$ 2,973	$ 2,374
Stock-based compensation expense for non-employee awards	(37)	12
Total stock-based compensation expense	2,936	2,386
Research and Development Expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for employee awards	1,681	1,284
General and Administrative Expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for employee awards	$ 1,292	$ 1,090