Net Loss Per Share - Antidilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Options to purchase common stock
Net Loss Per Share
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in shares)	6,148,974	4,115,204	5,633,972	3,748,900
Warrants
Net Loss Per Share
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in shares)	631,588	703,127	631,588	886,320
Restricted stock units (RSUs)
Net Loss Per Share
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in shares)	0	0	37,700	0
Performance units
Net Loss Per Share
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in shares)	0	0	0	0