Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Costs and expenses:
Total stock-based compensation expense	$ 2,953	$ 1,907
Research and development [Member]
Costs and expenses:
Total stock-based compensation expense	1,691	1,073
General and administrative [Member]
Costs and expenses:
Total stock-based compensation expense	$ 1,262	$ 834