Comprehensive income (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,176,350	$ 1,238,358
Current-period other comprehensive income (loss)	(1,509)	862
Reclassified from accumulated OCI into income	(2,429)	(7,312)
Income tax expense (benefit)	(2,268)	355
Ending Balance	1,239,198	1,162,161
Currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(25,831)	(22,485)
Current-period other comprehensive income (loss)	6,507	(9,303)
Reclassified from accumulated OCI into income	0	0
Income tax expense (benefit)	0	0
Ending Balance	(19,324)	(31,788)
Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(85)	(1,308)
Current-period other comprehensive income (loss)	(317)	1,494
Reclassified from accumulated OCI into income	0	0
Income tax expense (benefit)	(45)	(11)
Ending Balance	(357)	197
Derivative instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	4,846
Current-period other comprehensive income (loss)	(7,699)
Reclassified from accumulated OCI into income	(2,429)
Income tax expense (benefit)	(2,223)
Ending Balance	(3,059)
Derivative instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		6,862
Current-period other comprehensive income (loss)		8,671
Reclassified from accumulated OCI into income		(7,312)
Income tax expense (benefit)		366
Ending Balance		7,855
Accumulated other comprehensive income/(loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(21,070)	(16,931)
Ending Balance	$ (22,740)	$ (23,736)