Stock-Based Compensation (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 3,868	$ 1,958	$ 8,677	$ 4,719
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)	1.90%
Risk-free interest rate, minimum (as a percent)		2.10%	1.80%	2.10%
Risk-free interest rate, maximum (as a percent)		2.20%	2.20%	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)	$ 1,715	$ 977	$ 4,228	$ 2,315
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 2,153	$ 981	$ 4,449	$ 2,404