Net Income (Loss) Per Share - Computation Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Net Loss Per Share
Net loss	$ (4,485)	$ (6,762)	$ (10,286)	$ (6,035)	$ (6,293)	$ (5,925)	$ (21,533)	$ (18,253)
Weighted average common shares outstanding - basic and diluted (in shares)	26,089			25,111			25,868	24,315
Net loss per share - basic and diluted (in dollars per share)	$ (0.17)			$ (0.24)			$ (0.83)	$ (0.75)