Pension and Other Postretirement Employee Benefit Plans - Defined Benefit Pension and OPEB Plans (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Retirement Benefits [Abstract]
Projected benefit obligations	$ (14,740)	$ (13,545)
Fair value of plan assets	22,013	19,890
Net funded status	7,273	6,345
Accumulated other comprehensive income/(loss)	$ (1,517)	$ (1,916)
Weighted-average actuarial assumptions used to value benefit obligations
Discount rate	5.16%	5.14%

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Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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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 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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