Summary of Significant Accounting Policies (Net Loss Per 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	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss available for common shareholders	$ (11,948)	$ 289	$ (8,459)	$ (7,132)	$ (8,119)	$ (5,762)	$ (5,656)	$ (5,949)	$ (27,250)	$ (25,486)
Dilutive incremental share effect from:
Weighted-average common shares outstanding									86,983	72,786
Net loss per share - basic and diluted	$ (0.13)	$ 0	$ (0.10)	$ (0.09)	$ (0.10)	$ (0.08)	$ (0.08)	$ (0.09)	$ (0.31)	$ (0.35)