Accumulated other comprehensive income (loss) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2016	$ 5,783,487
Balance as of September 30, 2017	6,797,602
Foreign currency translation adjustments
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2016	(825,354)
Current period other comprehensive income (loss) before reclassifications	262,350
Balance as of September 30, 2017	(563,004)
Pensions, net of tax
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2016	(7,673)
Current period other comprehensive income (loss) before reclassifications	(692)
Balance as of September 30, 2017	(8,365)
AOCI attributable to parent
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2016	(833,027)
Current period other comprehensive income (loss) before reclassifications	261,658
Balance as of September 30, 2017	$ (571,369)