Net Loss per Share - Anti-dilutive Securities (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Securities excluded from the calculation of diluted net loss per share attributable to common stockholders
Amount of antidilutive securities excluded from computation of earnings per share	9,045,489	3,818,670
Stock options to purchase common stock
Securities excluded from the calculation of diluted net loss per share attributable to common stockholders
Amount of antidilutive securities excluded from computation of earnings per share	2,948,279	2,602,641
Unvested restricted stock
Securities excluded from the calculation of diluted net loss per share attributable to common stockholders
Amount of antidilutive securities excluded from computation of earnings per share	1,445,817	1,061,879
Common stock warrants
Securities excluded from the calculation of diluted net loss per share attributable to common stockholders
Amount of antidilutive securities excluded from computation of earnings per share	4,646,393
Contingently issuable shares
Securities excluded from the calculation of diluted net loss per share attributable to common stockholders
Amount of antidilutive securities excluded from computation of earnings per share	5,000	154,150