Net Loss Per Share - Antidilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
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)	5,759,140	6,233,398	5,728,531	7,468,544
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
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)	120,532	605,415	120,533	605,415
Restricted stock units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
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)	262,002	108,940	295,491	117,253