Stock-Based Compensation - Valuation and Expenses Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 3,973	$ 2,527	$ 7,913	$ 4,808
Assumptions used for fair values of employee and director stock options granted
Risk-free interest rate, minimum (as a percent)			1.50%	1.80%
Risk-free interest rate, maximum (as a percent)			1.60%	2.20%
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	77.00%	67.00%
Expected stock price volatility, minimum (as a percent)			70.00%	67.00%
Expected stock price volatility, maximum (as a percent)			77.00%	69.00%
Expected option term	6 years	6 years	6 years	6 years
Risk-free interest rate (as a percent)	1.50%	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)	$ 2,008	$ 1,244	$ 3,824	$ 2,512
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,965	$ 1,283	$ 4,089	$ 2,296