Income taxes - (Benefit From) Provision for Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Domestic	$ 2,299	$ 0	$ 0
Foreign	5,721	1,256	5,274
Total current income tax	8,020	1,256	5,274
Deferred:
Domestic	(2,967)	0	0
Foreign	(6,472)	742	121
Deferred Income Tax Expense (Benefit)	(9,439)	742	121
Total (benefit from) provision for income tax	$ (1,419)	$ 1,998	$ 5,395