Warrants - Liability-Classified Warrants Outstanding (Details) - $ / shares	1 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Warrants
Warrants issued to purchase common stock (shares)	22,303,109	22,303,109		5,666,779
Warrant exercise price (in dollars per share)	$ 2.27	$ 2.27		$ 7.14
Increase (decrease) in warrants (shares)	72,014	72,014
Issued in March 2010 financing
Warrants
Warrants issued to purchase common stock (shares)	0	0		319,789
Warrant exercise price (in dollars per share)	$ 6.25	$ 6.25
Issued in June 2011 financing
Warrants
Warrants issued to purchase common stock (shares)	0	0		6,113
Warrant exercise price (in dollars per share)	$ 22.50	$ 22.50
Issued in August 2011 financing
Warrants
Warrants issued to purchase common stock (shares)	0	0		565,759
Warrant exercise price (in dollars per share)	$ 18.75	$ 18.75
Issued to placement agents in August 2011 financing
Warrants
Warrants issued to purchase common stock (shares)	0	0		50,123
Warrant exercise price (in dollars per share)	$ 13.635	$ 13.635
Issued in Series B and D Preferred Stock offerings
Warrants
Warrants issued to purchase common stock (shares)	1,389,194	1,389,194		1,970,594
Warrant exercise price (in dollars per share)	$ 6.25	$ 6.25
Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock (shares)	132,014	132,014		60,000
Warrant exercise price (in dollars per share)	$ 1.13625	$ 1.13625	$ 2.50
Issued with June 2012 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	1,125,578	1,125,578		1,125,578
Warrant exercise price (in dollars per share)	$ 2.50	$ 2.50
Issued in Series E Preferred Stock offering
Warrants
Warrants issued to purchase common stock (shares)	1,568,823	1,568,823		1,568,823
Warrant exercise price (in dollars per share)	$ 7.50	$ 7.50
Issued with September 2016 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	18,087,500	18,087,500		0
Warrant exercise price (in dollars per share)	$ 1.50	$ 1.50