Summary of significant accounting policies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 3,148	$ 2,260	$ 9,676	$ 10,523
Accumulated loss	(513,618)		(513,618)		$ (475,318)
ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated loss	24,900		24,900
Deferred Revenue	(24,900)		(24,900)
Before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 1,000		$ 3,200