Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	$ 4,004,268	$ 3,793,637
Amounts reclassified from accumulated other comprehensive loss	450
Balance, net of tax, at end of year	3,911,790	4,004,268	$ 3,793,637
Accumulated other comprehensive loss
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(23,216)	(12,569)	(8,556)
Net current period other comprehensive income (loss), net of tax	574	(10,647)	(4,013)
Amounts reclassified from accumulated other comprehensive loss	450	0	0
Balance, net of tax, at end of year	(22,192)	(23,216)	(12,569)
Foreign currency translation adjustment
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(22,274)	(11,834)	(8,118)
Net current period other comprehensive income (loss), net of tax	4,057	(10,440)	(3,716)
Balance, net of tax, at end of year	(18,217)	(22,274)	(11,834)
Minimum pension liability
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(150)	334	(210)
Net current period other comprehensive income (loss), net of tax	2,869	(484)	544
Balance, net of tax, at end of year	2,719	(150)	334
Cash flow hedge
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(792)	(1,069)	(228)
Net current period other comprehensive income (loss), net of tax	(6,352)	277	(841)
Amounts reclassified from accumulated other comprehensive loss	450
Balance, net of tax, at end of year	$ (6,694)	$ (792)	$ (1,069)