Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Details
Revenue	$ 2,560	$ 6,086	$ 1,453	$ 792	$ 2,905	$ 4,000	$ 4,155	$ 3,701	$ 10,891	$ 14,761	$ 9,188
Gross profit	10	243	508	254	505	1,207	1,563	1,112	1,015	4,387	2,008
Net loss	$ (7,861)	$ (5,241)	$ (5,494)	$ (5,647)	$ (5,370)	$ (4,070)	$ (3,476)	$ (3,556)	$ (24,243)	$ (16,472)	$ (14,542)
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)