Accumulated other comprehensive loss - Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gains and losses on cash flow hedges:
Other comprehensive income/(loss)	$ (76,728)	$ (76,621)	$ 146,452
Foreign Currency Items
Gains and losses on cash flow hedges:
Other comprehensive income/(loss) before reclassifications, Before Tax Amount	9,108	(53,504)
Other comprehensive income/(loss) before reclassifications, Tax (Expense) Benefit	0	0
Other comprehensive income/(loss) before reclassifications	9,108	(53,504)
Other comprehensive income/(loss), Before Tax Amount	9,108	(52,607)
Other comprehensive income/(loss), Tax (Expense) Benefit	0	0
Other comprehensive income/(loss)	9,108	(52,607)
Foreign Currency Items | Net Income
Gains and losses on cash flow hedges:
Amounts reclassified from accumulated other comprehensive income/(loss), Before Tax Amount	0	897
Amounts reclassified from accumulated other comprehensive income/(loss), Tax (Expense) Benefit	0	0
Amounts reclassified from accumulated other comprehensive income/(loss)	0	897
Defined Benefit Pension Items
Gains and losses on cash flow hedges:
Other comprehensive income/(loss) before reclassifications, Before Tax Amount	(147,948)	(63,259)
Other comprehensive income/(loss) before reclassifications, Tax (Expense) Benefit	36,455	13,027
Other comprehensive income/(loss) before reclassifications	(111,493)	(50,232)
Other comprehensive income/(loss), Before Tax Amount	(115,189)	(25,584)
Other comprehensive income/(loss), Tax (Expense) Benefit	28,156	5,340
Other comprehensive income/(loss)	(87,033)	(20,244)
Defined Benefit Pension Items | Net Income
Gains and losses on cash flow hedges:
Amounts reclassified from accumulated other comprehensive income/(loss), Before Tax Amount	32,759	37,675
Amounts reclassified from accumulated other comprehensive income/(loss), Tax (Expense) Benefit	(8,299)	(7,687)
Amounts reclassified from accumulated other comprehensive income/(loss)	24,460	29,988
Gains and Losses on Cash Flow Hedges
Gains and losses on cash flow hedges:
Other comprehensive income/(loss) before reclassifications, Before Tax Amount	2,711	(2,096)
Other comprehensive income/(loss) before reclassifications, Tax (Expense) Benefit	(650)	716
Other comprehensive income/(loss) before reclassifications	2,061	(1,380)
Other comprehensive income/(loss), Before Tax Amount	2,711	(2,293)
Other comprehensive income/(loss), Tax (Expense) Benefit	(676)	679
Other comprehensive income/(loss)	2,035	(1,614)
Gains and Losses on Cash Flow Hedges | Fixed assets
Gains and losses on cash flow hedges:
Amounts reclassified from accumulated other comprehensive income/(loss), Before Tax Amount	(107)	(305)
Amounts reclassified from accumulated other comprehensive income/(loss), Tax (Expense) Benefit	0	0
Amounts reclassified from accumulated other comprehensive income/(loss)	(107)	(305)
Gains and Losses on Cash Flow Hedges | Net Income
Gains and losses on cash flow hedges:
Amounts reclassified from accumulated other comprehensive income/(loss), Before Tax Amount	107	108
Amounts reclassified from accumulated other comprehensive income/(loss), Tax (Expense) Benefit	(26)	(37)
Amounts reclassified from accumulated other comprehensive income/(loss)	81	71
Accumulated Other Comprehensive Loss
Gains and losses on cash flow hedges:
Other comprehensive income/(loss), Before Tax Amount	(103,370)	(80,484)
Other comprehensive income/(loss), Tax (Expense) Benefit	27,480	6,019
Other comprehensive income/(loss)	$ (75,890)	$ (74,465)	$ 145,347