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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic and diluted net loss per share of common stock:
Net loss	$ (4,738)	$ (4,170)	$ (4,552)	$ (5,438)	$ (6,014)	$ (6,464)	$ (8,949)	$ (7,216)	$ (18,898)	$ (28,643)	$ (24,850)
Weighted average shares of common stock outstanding (in shares)									23,540,748	19,498,143	14,919,783
Net loss per share of common stock-basic and diluted (in dollars per share)	$ (0.12)	$ (0.15)	$ (0.21)	$ (0.26)	$ (0.31)	$ (0.33)	$ (0.46)	$ (0.37)	$ (0.80)	$ (1.47)	$ (1.67)
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									3,994,120	2,434,944	1,799,226
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)									239,800	195,900
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)									3,754,320	2,239,044	1,799,226