Fair Value Measurements (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Changes in carrying amounts of the warrant liability
Beginning balance	$ 5,131
Net changes in valuation	(4,080)
Ending balance	1,051
Recurring [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	55,045	$ 31,159
Short-term investments	15,022	45,622
Warrant liabilities	(1,051)	(5,131)
Level I [Member] | Recurring [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	55,045	31,159
Short-term investments	15,022	45,622
Warrant liabilities	0	0
Level II [Member] | Recurring [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] | Recurring [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	(1,051)	$ (5,131)
Changes in carrying amounts of the warrant liability
Unrealized gain recognized	$ 4,100