Warrants - Liability-Classified Warrants Outstanding (Details) - $ / shares	Jun. 30, 2018	May 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)	6,436,304			5,452,923
Warrant exercise price (in dollars per share)	$ 9.83			$ 11.32
Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrant exercise price (in dollars per share)							$ 12.69
Issued in December 2017 Offering - common warrants
Warrants
Warrants issued to purchase common stock (shares)			409,091		82,118	2,727,273
Issued in December 2017 Offering - pre-funded warrants
Warrants
Warrants issued to purchase common stock (shares)	1,184,442			1,184,442		1,184,442
Warrant
Warrants
Warrants issued to purchase common stock (shares)	1,310,516			1,385,556
Warrant | Issued with June 2012 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	0			75,040
Warrant exercise price (in dollars per share)	$ 37.50
Warrant | Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock (shares)	104,676			104,676
Warrant exercise price (in dollars per share)	$ 112.50
Warrant | Issued with September 2016 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	1,205,840			1,205,840
Warrant exercise price (in dollars per share)	$ 22.50
Warrant
Warrants
Warrants issued to purchase common stock (shares)	5,125,788			4,067,367
Warrant | Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrants issued to purchase common stock (shares)	687,468			687,468
Warrant exercise price (in dollars per share)	$ 12.69
Warrant | Issued in December 2017 Offering - common warrants
Warrants
Warrants issued to purchase common stock (shares)	2,679,702			2,809,404
Warrant exercise price (in dollars per share)	$ 3.85		$ 3.85	$ 3.85		$ 3.85
Warrant | Issued in December 2017 Offering - underwriter warrants
Warrants
Warrants issued to purchase common stock (shares)	87,274			87,274
Warrant exercise price (in dollars per share)	$ 4.8125			$ 4.8125
Warrant | Issued in December 2017 Offering - pre-funded warrants
Warrants
Warrants issued to purchase common stock (shares)	0			483,221
Warrant exercise price (in dollars per share)	$ 0.05
Warrant | Issued in May 2018 Private Placement - common warrants
Warrants
Warrants issued to purchase common stock (shares)	1,528,668	1,528,668		0
Warrant exercise price (in dollars per share)	$ 2.86	$ 2.86
Warrant | Issued in May 2018 - underwriter warrants
Warrants
Warrants issued to purchase common stock (shares)	142,676	142,676		0
Warrant exercise price (in dollars per share)	$ 3.679	$ 3.679