Summary of significant accounting policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 3,478
Accumulated loss	(470,991)		$ (475,318)
ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated loss	24,900
Deferred Revenue	(24,900)
License revenues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	2,500
Before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue		$ 3,321
Before Topic 606 | License revenues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 1,100