Fair Value Measurements (Details) - Recurring [Member] - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	$ 38,437	$ 31,159
Short-term investments	15,002	45,622
Warrant liabilities	(4,567)	(5,131)
Level I [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	38,437	31,159
Short-term investments	15,002	45,622
Warrant liabilities	0	0
Level II [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	0	0
Short-term investments	0	0
Warrant liabilities	0	0
Level III [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	0	0
Short-term investments	0	0
Warrant liabilities	(4,567)	$ (5,131)
Unrealized gain recognized	$ 600