Accumulated other comprehensive loss - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification upon sale of operating unit	$ 750	$ (153)	$ 938
Cash flows from derivative financial instruments related to interest payments made for the hedged debt instrument	33,042	32,455	$ 29,155
Total reclassifications for the period	1,759	2,733
Foreign currency translation adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications for the period	0	692
Derivative financial instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications for the period	1,759	2,041
Reclassification out of AOCI \| Foreign currency translation adjustments \| Gain on disposal of property, plant and equipment and equity method investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification upon sale of operating unit	0	692
Reclassification out of AOCI \| Derivative financial instruments \| Interest expense
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Cash flows from derivative financial instruments related to interest payments made for the hedged debt instrument	$ 1,759	$ 2,041

Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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