Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock-based compensation expense
Stock-based compensation expense	$ 7,431	$ 6,030	$ 3,322
Research and development
Stock-based compensation expense
Stock-based compensation expense	4,412	3,220	2,485
General and administrative
Stock-based compensation expense
Stock-based compensation expense	3,019	2,810	837
Common stock options
Stock-based compensation expense
Stock-based compensation expense	7,076	5,757	3,082
Restricted stock
Stock-based compensation expense
Stock-based compensation expense	$ 355	$ 273	$ 240