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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss available for common shareholders - basic and diluted	$ (7,861)	$ (5,241)	$ (5,494)	$ (5,647)	$ (5,370)	$ (4,070)	$ (3,476)	$ (3,556)	$ (24,243)	$ (16,472)	$ (14,542)
Denominator:
Weighted-average shares outstanding - basic and diluted									72,786	51,958	46,540
Net loss per share - basic and diluted	$ (0.10)	$ (0.07)	$ (0.08)	$ (0.08)	$ (0.09)	$ (0.08)	$ (0.07)	$ (0.07)	$ (0.33)	$ (0.32)	$ (0.31)