Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - Beginning of period			$ 3,895,072
Amounts reclassified from accumulated other comprehensive income (loss)				$ 0
Balance - End of period	$ 3,809,136	$ 4,211,431	3,809,136	4,211,431
Accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - Beginning of period	9,405	(22,453)	(22,192)	(23,216)
Other comprehensive income (loss), net of tax	640	2,529	31,202	3,292
Amounts reclassified from accumulated other comprehensive income (loss)	(186)		849
Balance - End of period	9,859	(19,924)	9,859	(19,924)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - Beginning of period	(16,380)	(21,677)	(18,217)	(22,274)
Other comprehensive income (loss), net of tax	(2,737)	1,489	(900)	2,086
Amounts reclassified from accumulated other comprehensive income (loss)			0
Balance - End of period	(19,117)	(20,188)	(19,117)	(20,188)
Minimum pension liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - Beginning of period	2,681	(82)	2,719	(150)
Other comprehensive income (loss), net of tax	342	1,184	304	1,252
Amounts reclassified from accumulated other comprehensive income (loss)			0
Balance - End of period	3,023	1,102	3,023	1,102
Cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - Beginning of period	23,104	(694)	(6,694)	(792)
Other comprehensive income (loss), net of tax	3,035	(144)	31,798	(46)
Amounts reclassified from accumulated other comprehensive income (loss)	(186)		849
Balance - End of period	$ 25,953	$ (838)	$ 25,953	$ (838)