Warrants - Liability-Classified Warrants Outstanding (Details) - $ / shares	Mar. 31, 2018	Jan. 06, 2018	Dec. 31, 2017	Dec. 08, 2017	Dec. 07, 2017	Mar. 31, 2017
Warrants
Warrants issued to purchase common stock (shares)	24,848,061		27,264,165
Warrant exercise price (in dollars per share)	$ 2.48		$ 2.26
Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrant exercise price (in dollars per share)						$ 2.54
Issued in December 2017 Common Stock Offering - common warrants
Warrants
Warrants issued to purchase common stock (shares)		2,045,454		410,586	13,636,364
Issued in December 2017 Common Stock Offering - pre-funded warrants
Warrants
Warrants issued to purchase common stock (shares)	5,922,208		5,922,208		5,922,208
Warrant
Warrants
Warrants issued to purchase common stock (shares)	6,927,413		6,927,413
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)	$ 7.5
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)	$ 22.5
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)	$ 4.5
Warrant
Warrants
Warrants issued to purchase common stock (shares)	17,920,648		20,336,752
Warrant | Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrants issued to purchase common stock (shares)	3,437,334		3,437,334
Warrant exercise price (in dollars per share)	$ 2.537730
Warrant | Issued in December 2017 Common Stock Offering - common warrants
Warrants
Warrants issued to purchase common stock (shares)	14,046,950		14,046,950
Warrant exercise price (in dollars per share)	$ 0.77	$ 0.77	$ 0.77		$ 0.77
Warrant | Issued in December 2017 Common Stock Offering - underwriter warrants
Warrants
Warrants issued to purchase common stock (shares)	436,364		436,364
Warrant exercise price (in dollars per share)	$ 0.9625		$ 0.9625
Warrant | Issued in December 2017 Common Stock Offering - pre-funded warrants
Warrants
Warrants issued to purchase common stock (shares)	0		2,416,104
Warrant exercise price (in dollars per share)	$ 0.01