Warrants - Outstanding Liability Classified Warrants to Purchase Common Stock (Details) - $ / shares	Dec. 31, 2018	Jul. 31, 2018	May 31, 2018	Jan. 06, 2018	Dec. 31, 2017	Dec. 11, 2017	Dec. 08, 2017	Dec. 07, 2017	Mar. 31, 2017	Sep. 30, 2016
Class of Warrant or Right [Line Items]
Number of warrants (shares)	7,392,966				5,452,923
Warrants exercise price (usd per share)	$ 7.36				$ 11.32
Issued with June 2012 Convertible Notes
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0				75,040
Warrants exercise price (usd per share)	$ 37.50
Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0				104,676
Warrants exercise price (usd per share)	$ 112.50
Issued with September 2016 Convertible Notes
Class of Warrant or Right [Line Items]
Number of warrants (shares)	1,205,840				1,205,840					1,205,840
Warrants exercise price (usd per share)	$ 22.50									$ 22.50
Issued in 2017 Series A Preferred Stock Offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)									687,468
Warrants exercise price (usd per share)									$ 12.69
Issued in 2017 Common Stock Offering - common warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)				409,091	82,118		82,118	2,809,404
Issued in 2017 Common Stock Offering - pre-funded warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)					1,184,422			1,184,422
Warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	6,187,126				4,067,367
Warrants | Issued in 2017 Series A Preferred Stock Offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	687,468				687,468
Warrants exercise price (usd per share)	$ 12.69
Warrants | Issued in 2017 Common Stock Offering - common warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	2,679,702				2,809,404
Warrants exercise price (usd per share)	$ 3.85			$ 3.85	$ 3.85	$ 3.85		$ 3.85
Warrants | Issued in 2017 Common Stock Offering - underwriter warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	87,274				87,274		87,274
Warrants exercise price (usd per share)	$ 4.8125				$ 4.8125	4.8125	$ 4.8125
Warrants | Issued in 2017 Common Stock Offering - pre-funded warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0				483,221
Warrants exercise price (usd per share)	$ 0.05					$ 0.05
Warrants | Issued in May 2018 Private Placement - common warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	1,528,668				0
Warrants exercise price (usd per share)	$ 2.86
Warrants | Issued in May 2018 Registered Direct Offering - underwriter warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	142,676		142,676		0
Warrants exercise price (usd per share)	$ 3.679		$ 3.679
Warrants | Issued in July 2018 Private Placement - common warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	958,152	958,152			0
Warrants exercise price (usd per share)	$ 2.70	$ 2.70
Warrants | Issued in July 2018 Registered Direct Offering - underwriter warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	103,186	103,186			0
Warrants exercise price (usd per share)	$ 3.464	$ 3.464
Warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	1,205,840				1,385,556