Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,250	$ 708
Stock options
Assumptions used for fair values of employee and director stock options granted
Expected stock price volatility (as a percent)	71.00%	72.00%
Expected option term	6 years	6 years
Risk-free interest rate (as a percent)	2.20%	1.40%
Expected dividend yield (as a percent)	0.00%	0.00%
Research and development
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	591	441
General and administrative
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 659	$ 267