Warrants - Outstanding Liability Classified Warrants to Purchase Common Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 08, 2017	Dec. 07, 2017	Mar. 31, 2017	Sep. 30, 2016
Class of Warrant or Right [Line Items]
Number of warrants (shares)	27,264,165	6,998,843
Warrants exercise price (usd per share)	$ 2.26	$ 5.98
Increase in number of warrant shares due to anti-dilution provisions (shares)	80,197	154,288
Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0	71,430
Warrants exercise price (usd per share)	$ 2.10
Issued with June 2012 Convertible Notes
Class of Warrant or Right [Line Items]
Number of warrants (shares)	375,194	375,194
Warrants exercise price (usd per share)	$ 7.50
Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	523,045	523,045
Warrants exercise price (usd per share)	$ 22.50
Issued with September 2016 Convertible Notes
Class of Warrant or Right [Line Items]
Number of warrants (shares)	6,029,174	6,029,174				6,029,174
Warrants exercise price (usd per share)	$ 4.50					$ 4.50
Issued in 2017 Series A Preferred Stock Offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)					3,437,334
Warrants exercise price (usd per share)					$ 2.54
Issued in 2017 Common Stock Offering - common warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)			410,586	13,636,364
Issued in 2017 Common Stock Offering - pre-funded warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	5,922,208			5,922,208
Warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	20,336,752	0
Warrants | Issued in 2017 Series A Preferred Stock Offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	3,437,334	0
Warrants exercise price (usd per share)	$ 2.54
Warrants | Issued in 2017 Common Stock Offering - common warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	14,046,950	0
Warrants exercise price (usd per share)	$ 0.77			$ 0.77
Warrants | Issued in 2017 Common Stock Offering - underwriter warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	436,364	0
Warrants exercise price (usd per share)	$ 0.9625
Warrants | Issued in 2017 Common Stock Offering - pre-funded warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	2,416,104	0
Warrants exercise price (usd per share)	$ 0.01
Warrants
Class of Warrant or Right [Line Items]
Number of warrants (shares)	6,927,413	6,998,843
Increase in number of warrant shares due to anti-dilution provisions (shares)	80,197
Minimum | Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Warrants exercise price (usd per share)	$ 2.10	$ 2.50
Maximum | Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Warrants exercise price (usd per share)	$ 0.77	$ 2.10