Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 1,511	$ 676	$ 2,761	$ 1,384
Employee Consultant and Non Employee Director Stock Option [Member]
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 option term	6 years	6 years	6 years	6 years
Risk-free interest rate (as a percent)	2.20%		2.20%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%	0.00%
Expected stock price volatility, minimum (as a percent)			71.00%
Expected stock price volatility, maximum (as a percent)			72.00%
Risk-free interest rate, minimum (as a percent)		1.20%		1.20%
Risk-free interest rate, maximum (as a percent)		1.80%		1.80%
Research and Development Expense [Member]
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	747	422	1,338	863
General and Administrative Expense [Member]
Valuation and Expenses Information
Total stock-based compensation expense (in dollars)	$ 764	$ 254	$ 1,423	$ 521