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Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (19,284)	$ (18,680)	$ (19,451)	$ (16,089)	$ (15,767)	$ (15,343)	$ (57,415)	$ (47,199)
Denominator:
Weighted-average common shares outstanding - basic and diluted (in shares)	50,364,649			39,900,551			48,270,771	38,598,304
Net loss per share - basic and diluted (in dollars per share)	$ (0.38)			$ (0.40)			$ (1.19)	$ (1.22)
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	11,243,587			10,110,852			11,243,587	10,110,852
Shares Issuable upon Conversion of the 2021 Convertible Notes [Member]
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	1,620,948			1,620,948			1,620,948	1,620,948
Warrants Exercisable for Common Shares [Member]
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	14,102			14,102			14,102	14,102
Options to Purchase Common Shares [Member]
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	9,608,537			8,475,802			9,608,537	8,475,802