Warrants - Liability-Classified Warrants Outstanding (Details) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
Warrants
Warrants issued to purchase common stock (shares)	10,411,177	6,998,843
Warrant exercise price (in dollars per share)	$ 4.85	$ 5.98
Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrant exercise price (in dollars per share)	$ 2.54
Warrant
Warrants
Warrants issued to purchase common stock (shares)	6,973,843	6,998,843
Warrant | Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock (shares)	46,430	71,430
Warrant exercise price (in dollars per share)	$ 2.10
Warrant | Issued with June 2012 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	375,194	375,194
Warrant exercise price (in dollars per share)	$ 7.50
Warrant | Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock (shares)	523,045	523,045
Warrant exercise price (in dollars per share)	$ 22.50
Warrant | Issued with September 2016 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	6,029,174	6,029,174
Warrant exercise price (in dollars per share)	$ 4.50
Warrant
Warrants
Warrants issued to purchase common stock (shares)	3,437,334	0
Warrant | Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrants issued to purchase common stock (shares)	3,437,334	0
Warrant exercise price (in dollars per share)	$ 2.54