Benefit Plans - Changes in benefit obligations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Change in benefit obligation
Benefit obligation, beginning of year	$ 6,815	$ 6,591
Service cost	113	114	$ 113
Interest cost	242	264	286
Participant contributions	0	0
Actuarial (gain) loss	245	(395)
Benefits paid	(646)	(553)
Plan amendments	0	0
Translation adjustment and other	(55)	4
Benefit obligation, end of year	6,224	6,815	6,591
Postretirement benefits
Change in benefit obligation
Benefit obligation, beginning of year	386	373
Service cost	7	8	9
Interest cost	13	15	17
Participant contributions	13	12
Actuarial (gain) loss	3	(8)
Benefits paid	(35)	(35)
Plan amendments	0	0
Translation adjustment and other	(6)	5
Benefit obligation, end of year	$ 375	$ 386	$ 373

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 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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- Definition

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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No definition available.

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

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- Definition

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 in terms of existing plan or initiation of new 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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