Employee benefit plans - Plans' Obligation and Assets (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in Plan Assets
Fair value of plan assets at January 1	$ 1,329,751
Fair value of plan assets at December 31	1,696,401	$ 1,329,751
Retirement Plan
Change in Benefit Obligation
Benefit obligation at January 1	1,684,277	1,837,938
Service cost	3,968	18,652	$ 18,543
Interest cost	57,348	54,970	55,873
Plan participant contributions	224	429
Plan amendments	1,343	155
Actuarial loss/(gain)	316,547	(115,153)
Benefits paid	(92,636)	(93,053)
Impact of foreign exchange rates	11,952	(21,636)
Effect of settlements	(8,101)	(2,210)
Other	0	(253)
Acquisitions	1,275	4,438
Benefit obligation at December 31	1,976,197	1,684,277	1,837,938
Change in Plan Assets
Fair value of plan assets at January 1	1,318,832	1,494,713
Actual return on plan assets	242,823	(78,447)
Company contributions	215,979	24,524
Plan participant contributions	224	429
Benefits paid	(92,636)	(93,053)
Impact of foreign exchange rates	12,869	(22,380)
Effect of settlements	(8,101)	(2,210)
Expenses paid	(7,084)	(6,670)
Acquisitions	614	1,926
Fair value of plan assets at December 31	1,683,520	1,318,832	1,494,713
Funded Status of the Plans	(292,677)	(365,445)
Retiree Health and Life Insurance Plans
Change in Benefit Obligation
Benefit obligation at January 1	14,048	15,691
Service cost	308	297	313
Interest cost	467	452	463
Plan participant contributions	680	620
Plan amendments	0	0
Actuarial loss/(gain)	589	(398)
Benefits paid	(1,621)	(2,569)
Impact of foreign exchange rates	24	(45)
Effect of settlements	0	0
Other	0	0
Acquisitions	0	0
Benefit obligation at December 31	14,495	14,048	15,691
Change in Plan Assets
Fair value of plan assets at January 1	10,919	27,177
Actual return on plan assets	2,327	(915)
Company contributions	682	(13,302)
Plan participant contributions	680	620
Benefits paid	(1,621)	(2,569)
Impact of foreign exchange rates	0	0
Effect of settlements	0	0
Expenses paid	(106)	(92)
Acquisitions	0	0
Fair value of plan assets at December 31	12,881	10,919	$ 27,177
Funded Status of the Plans	$ (1,614)	$ (3,129)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of administration expense of defined benefit plan which decreases plan assets. Excludes plan administration expense paid by employer.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of contributions received by defined benefit plan from participant which increase benefit obligation.

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Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement.

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Amount of increase in benefit obligation of defined benefit plan from business combination.

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Amount of increase in plan assets of defined benefit plan from business combination.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of foreign currency translation gain (loss) which (increases) decreases benefit obligation of defined benefit plan.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change, classified as other.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of contributions received by defined benefit plan from participant which increases plan assets.

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Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract.

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