Income taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax expense:
Federal	$ 5,540	$ 1,154	$ 4,718
State	1,089	814	1,447
Foreign	6,622	4,625	5,983
Current income tax expense:	13,251	6,593	12,148
Deferred income tax expense (benefit):
Federal	43	4,088	6,262
State	(864)	(254)	(1,347)
Foreign	1,077	4,494	2,542
Deferred income tax expense (benefit):	256	8,328	7,457
Total income tax expense	$ 13,507	$ 14,921	$ 19,605