Employee Pension and Other Postretirement Benefits - Changes in Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Accumulated benefit obligation	$ 3,521	$ 3,038
Change in benefit obligation
Benefit obligation, beginning of year	3,047	3,242
Service cost	76	80	$ 71
Interest cost	117	100	$ 100
Plan participants' contributions	0	0
Actuarial loss/(gain)	$ (439)	$ 246
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.00%	4.00%	3.00%
Effect of plan amendments	$ 0	$ 0
Medicare Part D subsidy	0	0
Benefits paid	(143)	(129)
Benefit obligation, end of year	3,536	3,047	$ 3,242
Fair value of plan assets, end of year	4,140	3,529	3,854
Over (Under) funded status	604	482
Other Benefits
Change in benefit obligation
Benefit obligation, beginning of year	242	265
Service cost	3	4	6
Interest cost	9	8	$ 8
Plan participants' contributions	4	4
Actuarial loss/(gain)	$ (14)	$ 21
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.00%	4.00%	3.00%
Effect of plan amendments	$ 0	$ 0
Medicare Part D subsidy	0	1
Benefits paid	(21)	(19)
Benefit obligation, end of year	251	242	$ 265
Fair value of plan assets, end of year	291	255	$ 288
Over (Under) funded status	$ 40	$ 13

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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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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Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit 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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Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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 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 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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