Accumulated other comprehensive loss - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification upon sale of operating unit	$ (153)	$ 938	$ 20,275
Cash flows from derivative financial instruments related to interest payments made for the hedged debt instrument	32,455	29,155	$ 32,101
Total reclassifications for the period	2,733	2,809
Foreign currency translation adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications for the period	(692)	0
Derivative financial instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications for the period	(2,041)	(2,809)
Reclassification out of AOCI | Foreign currency translation adjustments | (Loss)/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	692	0
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	$ 2,041	$ 2,809