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Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (18,680)	$ (19,451)	$ (15,767)	$ (15,343)	$ (38,131)	$ (31,110)
Denominator:
Weighted-average common shares outstanding - basic and diluted (in shares)	49,267,247		39,483,006		47,206,480	37,954,972
Net loss per share - basic and diluted	$ (0.38)		$ (0.40)		$ (0.81)	$ (0.82)
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	11,067,625		10,505,054		11,067,625	10,505,054
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)	9,432,575		8,870,004		9,432,575	8,870,004