Warrants - Outstanding Liability Classified Warrants to Purchase Common Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Class of Warrant or Right [Line Items]
Number of warrants (shares)	20,996,189		5,666,779
Warrants exercise price (usd per share)	$ 1.99		$ 7.14
Increase in number of warrant shares due to anti-dilution provisions (shares)	154,288
Issued in March 2010 financing
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0		319,789
Warrants exercise price (usd per share)	$ 6.25
Issued in June 2011 financing
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0		6,113
Warrants exercise price (usd per share)	$ 22.50
Issued in August 2011 financing
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0		565,759
Warrants exercise price (usd per share)	$ 18.75
Issued to placement agents in August 2011 financing
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0		50,123
Warrants exercise price (usd per share)	$ 13.635
Issued in Series B and D Preferred Stock offerings
Class of Warrant or Right [Line Items]
Number of warrants (shares)	0		1,970,594
Warrants exercise price (usd per share)	$ 6.25
Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	214,288		60,000
Warrants exercise price (usd per share)	$ 0.70		$ 2.50
Issued with June 2012 Convertible Notes
Class of Warrant or Right [Line Items]
Number of warrants (shares)	1,125,578		1,125,578
Warrants exercise price (usd per share)	$ 2.50
Issued in Series E Preferred Stock offering
Class of Warrant or Right [Line Items]
Number of warrants (shares)	1,568,823		1,568,823
Warrants exercise price (usd per share)	$ 7.50
Issued with September 2016 Convertible Notes
Class of Warrant or Right [Line Items]
Number of warrants (shares)	18,087,500		0
Warrants exercise price (usd per share)	$ 1.50	$ 1.50