Net Loss Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ 5,242	$ (1,069)	$ (4,651)	$ (7,659)	$ (5,407)	$ (2,388)	$ (9,778)	$ 287	$ (8,137)	$ (17,286)	$ (19,566)
Less: earnings attributable to convertible preferred stock									0	0	7,579
Numerator of basic and diluted EPS									$ (8,137)	$ (17,286)	$ (27,145)
Denominator for basic and diluted EPS:
Weighted-average common shares outstanding (in shares)									40,242	33,355	23,093
Net loss per share attributable to common shareholders (Basic and Diluted) (in dollars per share)					$ (0.16)	$ (0.07)	$ (0.31)	$ 0.01		$ (0.52)	$ (1.18)
Aggregate number of common equivalent shares (related to options, warrants and preferred stock) that have been excluded from computations of diluted net loss per common share (in shares)									4,800	5,400	5,300