Fair Value Measurements (Details) (Recurring [Member], USD $)	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	$ 49,281,000	$ 10,281,000
Short-term investments	50,602,000	27,085,000
Warrant liabilities	(11,948,000)	(24,182,000)
Level I [Member]
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	49,281,000	10,281,000
Short-term investments	50,602,000	27,085,000
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	(11,948,000)	(24,182,000)
Unrealized gain recognized	$ 12,200,000