Retirement Benefits - Summary of Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation as of the beginning of year	$ 10,677	$ 14,236
Service cost	155	236	$ 294
Interest cost	578	439	406
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0	0
Actuarial loss (gain)(a)	406	(3,379)
Plan amendments	4	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	(42)	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(790)	(855)
Net benefit obligation as of the end of year	10,988	10,677	14,236
Fair value of net plan assets as of the beginning of year	9,521	12,165
Actual return on plan assets	638	(2,359)
Employer contributions	75	570
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	0	0
Defined Benefit Plan, Plan Assets, Benefits Paid	(790)	(855)
Defined Benefit Plan, Plan Assets, Payment for Settlement	(42)	0
Fair value of net plan assets as of the end of year	9,402	9,521	12,165
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation as of the beginning of year	1,884	2,502
Service cost	26	41	51
Interest cost	101	76	69
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	27	26
Actuarial loss (gain)(a)	55	(604)
Plan amendments	0	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	0	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(185)	(157)
Net benefit obligation as of the end of year	1,908	1,884	2,502
Fair value of net plan assets as of the beginning of year	1,351	1,665
Actual return on plan assets	108	(225)
Employer contributions	54	42
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	27	26
Defined Benefit Plan, Plan Assets, Benefits Paid	(185)	(157)
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	0
Fair value of net plan assets as of the end of year	$ 1,355	$ 1,351	$ 1,665