Comprehensive income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,238,358	$ 1,128,021
Current-period other comprehensive (loss) income	(15,678)	993
Reclassified from accumulated OCI into income	10,844	1,386
Income tax benefit (expense)	695	(2,801)
Ending balance	1,176,350	1,238,358
Currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(22,485)	(12,717)
Current-period other comprehensive (loss) income	(3,346)	(9,768)
Reclassified from accumulated OCI into income	0	0
Income tax benefit (expense)	0	0
Ending balance	(25,831)	(22,485)
Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,308)	(782)
Current-period other comprehensive (loss) income	1,141	(378)
Reclassified from accumulated OCI into income	0	0
Income tax benefit (expense)	82	(148)
Ending balance	(85)	(1,308)
Derivative instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6,862
Current-period other comprehensive (loss) income	(13,473)
Reclassified from accumulated OCI into income	10,844
Income tax benefit (expense)	613
Ending balance	4,846	6,862
Derivative instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6,862	(3,010)
Current-period other comprehensive (loss) income		11,139
Reclassified from accumulated OCI into income		1,386
Income tax benefit (expense)		(2,653)
Ending balance		6,862
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(16,931)	(16,509)
Ending balance	$ (21,070)	$ (16,931)