Comprehensive income (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,128,021
Current-period other comprehensive (loss) income	(791)	$ 10,148
Reclassified from accumulated OCI into income	2,495	(792)
Income tax expense	1,760	(2,146)
Ending Balance	1,165,678
Currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,717)	(37,174)
Current-period other comprehensive (loss) income	(6,001)	15,664
Reclassified from accumulated OCI into income	0	0
Income tax expense	0	120
Ending Balance	(18,718)	(21,630)
Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(782)	(669)
Current-period other comprehensive (loss) income	(557)	26
Reclassified from accumulated OCI into income	0	0
Income tax expense	33	(17)
Ending Balance	(1,372)	(626)
Derivative instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3,010)	3,222
Current-period other comprehensive (loss) income	5,767	(5,542)
Reclassified from accumulated OCI into income	2,495	(792)
Income tax expense	1,727	(2,249)
Ending Balance	3,525	(863)
Accumulated other comprehensive income/(loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(16,509)	(34,621)
Ending Balance	$ (16,565)	$ (23,119)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.

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Amount before tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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