Income taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax expense:
Federal	$ 1,154	$ 4,718	$ 4,774
State	814	1,447	709
Foreign	4,625	5,983	5,967
Current income tax expense:	6,593	12,148	11,450
Deferred income tax expense (benefit):
Federal	4,088	6,262	(3,986)
State	(254)	(1,347)	(1,375)
Foreign	4,494	2,542	(439)
Deferred income tax expense (benefit):	8,328	7,457	(5,800)
Total income tax expense	$ 14,921	$ 19,605	$ 5,650