Stock-Based Compensation (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 2,527	$ 1,511	$ 4,808	$ 2,761
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	67.00%	72.00%
Expected stock price volatility, minimum (as a percent)			67.00%	71.00%
Expected stock price volatility, maximum (as a percent)			69.00%	72.00%
Expected option term	6 years	6 years	6 years	6 years
Risk-free interest rate (as a percent)	2.20%	2.20%		2.20%
Risk-free interest rate, minimum (as a percent)			1.80%
Risk-free interest rate, maximum (as a percent)			2.20%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,244	$ 747	$ 2,512	$ 1,338
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,283	$ 764	$ 2,296	$ 1,423