Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 2,016	$ 525	$ 3,400	$ 1,614
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	72.00%	71.00%	72.00%
Expected option term	6 years	6 years	6 years	6 years
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%	0.00%
Stock options | Minimum
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)				70.00%
Risk-free interest rate (as a percent)	1.90%	1.20%	1.20%	0.90%
Stock options | Maximum
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)				71.00%
Risk-free interest rate (as a percent)	2.00%	1.30%	2.00%	1.40%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	1,326	330	2,190	1,010
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 690	$ 195	$ 1,210	$ 604