Summary of Significant Accounting Policies - Loss Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted net loss per share of common stock:
Net loss	$ (10,113)	$ (11,110)	$ (9,504)	$ (5,999)	$ (4,738)	$ (4,170)	$ (4,552)	$ (5,438)	$ (36,726)	$ (18,898)	$ (28,643)
Weighted average shares of common stock outstanding (in shares)									40,895,406	23,540,748	19,498,143
Net loss per share of common stock-basic and diluted (in dollars per share)	$ (0.24)	$ (0.27)	$ (0.24)	$ (0.15)	$ (0.12)	$ (0.15)	$ (0.21)	$ (0.26)	$ (0.90)	$ (0.80)	$ (1.47)
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									5,056,609	3,994,120
Restricted stock
Basic and diluted net loss per share of common stock:
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									105,200	239,800
Stock options
Basic and diluted net loss per share of common stock:
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									4,951,409	3,754,320