Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	$ 3,793,637	$ 3,878,860
Balance, net of tax, at end of year	4,004,268	3,793,637	$ 3,878,860
Accumulated other comprehensive loss
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(12,569)	(8,556)	(617)
Net current period other comprehensive income (loss), net of tax	(10,647)	(4,013)	(7,939)
Balance, net of tax, at end of year	(23,216)	(12,569)	(8,556)
Foreign currency translation adjustment
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(11,834)	(8,118)	(617)
Net current period other comprehensive income (loss), net of tax	(10,440)	(3,716)	(7,501)
Balance, net of tax, at end of year	(22,274)	(11,834)	(8,118)
Minimum pension liability
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	334	(210)	0
Net current period other comprehensive income (loss), net of tax	(484)	544	(210)
Balance, net of tax, at end of year	(150)	334	(210)
Cash flow hedge
Accumulated Other Comprehensive Loss [Line Items]
Balance, net of tax, at beginning of year	(1,069)	(228)	0
Net current period other comprehensive income (loss), net of tax	277	(841)	(228)
Balance, net of tax, at end of year	$ (792)	$ (1,069)	$ (228)