Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Details
Revenue	$ 2,905	$ 4,000	$ 4,155	$ 3,701	$ 1,846	$ 2,398	$ 4,043	$ 901	$ 14,761	$ 9,188	$ 3,485
Gross profit	505	1,207	1,563	1,112	379	585	1,187	(143)	4,387	2,008	2,441
Net loss	$ (5,370)	$ (4,070)	$ (3,476)	$ (3,553)	$ (4,298)	$ (3,513)	$ (2,769)	$ (3,962)	$ (16,472)	$ (14,542)	$ (18,120)
Net loss per share - basic and diluted	$ (0.09)	$ (0.08)	$ (0.07)	$ (0.07)	$ (0.09)	$ (0.07)	$ (0.06)	$ (0.09)	$ (0.32)	$ (0.31)	$ (0.44)