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Net Loss Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Basic and diluted net loss per common share calculation:
Net loss	$ (14,443)	$ (12,501)	$ (13,209)	$ (13,073)	$ (26,944)	$ (26,282)
Weighted average common shares outstanding - basic and dilutive (in shares)	41,477		36,188		41,452	36,171
Net loss per common shares - basic and diluted (in dollars per share)	$ (0.35)		$ (0.37)		$ (0.65)	$ (0.73)
Anti-dilutive securities
Total (in shares)	6,325		5,017		6,325	5,017
Stock options outstanding
Anti-dilutive securities
Total (in shares)	5,690		4,818		5,690	4,818
Restricted stock units
Anti-dilutive securities
Total (in shares)	635		199		635	199