Warrants - Outstanding Liability Classified Warrants to Purchase Common Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Class of Warrant or Right [Line Items]
Warrants exercise price (usd per share)	$ 7.14	$ 7.08
Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	5,666,779	6,033,050
Issued in Series A, B and D Preferred Stock offerings (1) | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	1,970,594	2,247,118
Warrants exercise price (usd per share)	$ 6.25
Issued in Series A, B and D Preferred Stock offerings (1) | Warrant Liability | Maximum
Class of Warrant or Right [Line Items]
Expiration dates	Dec. 31, 2016
Issued in Series A, B and D Preferred Stock offerings (1) | Warrant Liability | Minimum
Class of Warrant or Right [Line Items]
Expiration dates	Oct. 13, 2015
Issued in March 2010 financing | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	319,789	393,416
Warrants exercise price (usd per share)	$ 6.25
Expiration dates	Mar. 04, 2016
Issued in June 2011 financing | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	6,113	6,113
Warrants exercise price (usd per share)	$ 22.50
Expiration dates	Jun. 16, 2016
Issued in August 2011 financing | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	565,759	565,759
Warrants exercise price (usd per share)	$ 18.75
Expiration dates	Aug. 22, 2016
Issued to placement agents in August 2011 financing | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	50,123	50,123
Warrants exercise price (usd per share)	$ 13.635
Expiration dates	Aug. 22, 2016
Issued in Series B, D and E Preferred Stock offerings (2) | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	60,000	76,120
Warrants exercise price (usd per share)	$ 2.50
Issued in Series B, D and E Preferred Stock offerings (2) | Warrant Liability | Maximum
Class of Warrant or Right [Line Items]
Expiration dates	Dec. 31, 2017
Issued in Series B, D and E Preferred Stock offerings (2) | Warrant Liability | Minimum
Class of Warrant or Right [Line Items]
Expiration dates	Nov. 17, 2015
Issued with Convertible Notes | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	1,125,578	1,125,578
Warrants exercise price (usd per share)	$ 2.50
Expiration dates	Jun. 01, 2018
Issued in Series E Preferred Stock offering | Warrant Liability
Class of Warrant or Right [Line Items]
Warrants issued to purchase common stock	1,568,823	1,568,823
Warrants exercise price (usd per share)	$ 7.50
Expiration dates	Dec. 13, 2018