Employee Postretirement Benefits (Principal Plans and All Other Pension Plans) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Principal Plans
Projected benefit obligation (PBO)	$ 3,406	$ 3,094
Accumulated benefit obligation (ABO)	3,406	3,094
Fair value of plan assets	3,303	2,936
All Other Pension Plans
Projected benefit obligation (PBO)	641	593
Accumulated benefit obligation (ABO)	561	521
Fair value of plan assets	500	462
Defined Benefit Pension Plans
Projected benefit obligation (PBO)	4,047	3,687	$ 4,296
Accumulated benefit obligation (ABO)	3,967	3,615
Fair value of plan assets	$ 3,803	$ 3,398	$ 3,897

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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