Warrants - Liability-Classified Warrants Outstanding (Details) - $ / shares	Jun. 30, 2016	Dec. 31, 2015
Warrants
Warrants issued to purchase common stock (shares)	5,340,877	5,666,779
Warrant exercise price (in dollars per share)	$ 7.18	$ 7.14
Issued in March 2010 financing
Warrants
Warrants issued to purchase common stock (shares)	0	319,789
Warrant exercise price (in dollars per share)	$ 6.25
Issued in June 2011 financing
Warrants
Warrants issued to purchase common stock (shares)	0	6,113
Warrant exercise price (in dollars per share)	$ 22.50
Issued in Series B and D Preferred Stock offerings
Warrants
Warrants issued to purchase common stock (shares)	1,970,594	1,970,594
Warrant exercise price (in dollars per share)	$ 6.25
Issued in August 2011 financing
Warrants
Warrants issued to purchase common stock (shares)	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 (shares)	50,123	50,123
Warrant exercise price (in dollars per share)	$ 13.635
Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock (shares)	60,000	60,000
Warrant exercise price (in dollars per share)	$ 2.50
Issued with Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	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 (shares)	1,568,823	1,568,823
Warrant exercise price (in dollars per share)	$ 7.50