Fair Value Measurements (Details) (Recurring [Member], USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Fair value measurements by level for assets and liabilities measured at fair value on a recurring basis [Abstract]
Cash equivalents	$ 57,238,000	$ 10,281,000
Short-term investments	50,585,000	27,085,000
Warrant liabilities	(9,479,000)	(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	57,238,000	10,281,000
Short-term investments	50,585,000	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
Unrealized gain recognized	14,700,000
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	$ (9,479,000)	$ (24,182,000)