Employee Postretirement Benefits (Principal Plans and All Other Pension Plans) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Principal Plans
Projected benefit obligation (PBO)	$ 3,629	$ 3,406
Accumulated benefit obligation (ABO)	3,629	3,406
Fair value of plan assets	3,627	3,303
All Other Pension Plans
Projected benefit obligation (PBO)	712	641
Accumulated benefit obligation (ABO)	619	561
Fair value of plan assets	566	500
Defined Benefit Pension Plans
Projected benefit obligation (PBO)	4,341	4,047	$ 3,687
Accumulated benefit obligation (ABO)	4,248	3,967
Fair value of plan assets	$ 4,193	$ 3,803	$ 3,398

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

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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 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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