Fair Value Measurements (Details) (Recurring [Member], USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	$ 27,429,000	$ 31,159,000
Short-term investments	37,102,000	45,622,000
Warrant liability	(7,002,000)	(5,131,000)
Level I [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	27,429,000	31,159,000
Short-term investments	37,102,000	45,622,000
Warrant liability	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 liability	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 liability	(7,002,000)	(5,131,000)
Unrealized loss recognized	$ (1,900,000)