Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Costs and expenses:
Total stock-based compensation expense	$ 9,200	$ 6,146	$ 5,338
Research and development [Member]
Costs and expenses:
Total stock-based compensation expense	5,064	3,204	2,892
General and administrative [Member]
Costs and expenses:
Total stock-based compensation expense	$ 4,136	$ 2,942	$ 2,446