Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Quarterly Financial Information Details
Revenue	$ 395	$ 639	$ 587	$ 1,469	$ 4,605	$ 1,190	$ 1,240	$ 1,851	$ 3,090	$ 8,886
Gross profit	395	639	588	70	1,179	(882)	(583)	608	1,692	322
Net loss	$ (3,570)	$ (2,826)	$ (2,304)	$ (4,934)	$ (3,284)	$ (6,141)	$ (8,990)	$ (8,068)	$ (13,634)	$ (26,483)
Net loss per share, basic and diluted	$ (0.02)	$ (0.02)	$ (0.02)	$ (0.04)	$ (0.03)	$ (0.05)	$ (0.08)	$ (0.08)	$ (0.10)	$ (0.24)