Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Costs and expenses:
Total stock-based compensation expense	$ 1,907	$ 1,329
Research and development [Member]
Costs and expenses:
Total stock-based compensation expense	1,073	685
General and administrative [Member]
Costs and expenses:
Total stock-based compensation expense	$ 834	$ 644