Accumulated other comprehensive income (loss) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	$ 7,067,009
Balance as of June 30, 2018	7,373,434
Foreign currency translation adjustments
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	(547,927)
Current period other comprehensive income	(112,829)
Balance as of June 30, 2018	(660,756)
Pensions, net of tax
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	(10,600)
Current period other comprehensive income	223
Balance as of June 30, 2018	(10,377)
AOCI attributable to parent
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	(558,527)
Current period other comprehensive income	(112,606)
Balance as of June 30, 2018	$ (671,133)