Quarterly financial information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Details
Revenue	$ 1,846	$ 2,398	$ 4,043	$ 901	$ 687	$ 968	$ 611	$ 1,219	$ 9,188	$ 3,485	$ 5,852
Gross margin	379	585	1,187	(143)	178	600	443	1,220	2,008	2,441	4,051
Net loss	$ (4,298)	$ (3,513)	$ (2,769)	$ (3,962)	$ (3,346)	$ (3,355)	$ (3,401)	$ (8,018)	$ (14,542)	$ (18,120)	$ (13,178)
Net loss per share - basic and diluted	$ (0.09)	$ (0.07)	$ (0.06)	$ (0.09)	$ (0.08)	$ (0.08)	$ (0.08)	$ (0.23)	$ (0.31)	$ (0.44)	$ (0.47)