Income taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current income tax expense:
Federal	$ 12,329	$ 5,540	$ 1,154
State	846	1,089	814
Foreign	4,851	6,622	4,625
Current income tax expense:	18,026	13,251	6,593
Deferred income tax expense (benefit):
Federal	6,631	43	4,088
State	150	(864)	(254)
Foreign	1,888	1,077	4,494
Deferred income tax expense (benefit):	8,669	256	8,328
Total income tax expense	$ 26,695	$ 13,507	$ 14,921