Warrants (Details) - $ / shares	Jun. 30, 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