Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Details
Revenue	$ 1,833	$ 11,572	$ 2,014	$ 2,188	$ 2,302	$ 5,425	$ 1,339	$ 568	$ 17,607	$ 9,634
Gross profit	(2,606)	8,927	333	315	(248)	(278)	346	(48)	6,969	(228)
Net income (loss)	$ (11,948)	$ 289	$ (8,459)	$ (7,132)	$ (8,119)	$ (5,762)	$ (5,656)	$ (5,949)	$ (27,250)	$ (25,486)
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)