Warrants (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Warrants
Warrants issued to purchase common stock	6,033,050	6,033,050
Issued in Series A, B and D Preferred Stock offerings
Warrants
Warrants issued to purchase common stock	2,247,118	2,247,118
Warrant Exercise Price (in dollars per share)	6.25
Issued in March 2010 financing
Warrants
Warrants issued to purchase common stock	393,416	393,416
Warrant Exercise Price (in dollars per share)	6.25
Issued in June 2011 financing
Warrants
Warrants issued to purchase common stock	6,113	6,113
Warrant Exercise Price (in dollars per share)	22.50
Issued in August 2011 financing
Warrants
Warrants issued to purchase common stock	565,759	565,759
Warrant Exercise Price (in dollars per share)	18.75
Issued to placement agents in August 2011 financing
Warrants
Warrants issued to purchase common stock	50,123	50,123
Warrant Exercise Price (in dollars per share)	13.635
Issued in Series B, D and E Preferred Stock offerings
Warrants
Warrants issued to purchase common stock	76,120	76,120
Warrant Exercise Price (in dollars per share)	2.50
Issued with Convertible Notes
Warrants
Warrants issued to purchase common stock	1,125,578	1,125,578
Warrant Exercise Price (in dollars per share)	2.50
Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock	1,568,823	1,568,823
Warrant Exercise Price (in dollars per share)	7.50