Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,958	$ 2,016	$ 4,719	$ 3,400
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	72.00%	72.00%		72.00%
Expected stock price volatility, minimum (as a percent)			71.00%
Expected stock price volatility, maximum (as a percent)			72.00%
Expected option term	6 years	6 years	6 years	6 years
Risk-free interest rate, minimum (as a percent)	2.10%	1.90%	2.10%	1.20%
Risk-free interest rate, maximum (as a percent)	2.20%	2.00%	2.20%	2.00%
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)	977	1,326	2,315	2,190
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 981	$ 690	$ 2,404	$ 1,210