Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Amortization of:
Net periodic benefit cost	$ (18)	$ 63	$ 91
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	155	236	294
Interest cost	578	439	406
Expected return on assets	(755)	(822)	(843)
Amortization of:
Prior service cost (credit)	2	2	2
Actuarial loss (gain)	166	295	399
Settlement and other charges	20	0	7
Net periodic benefit cost	166	150	265
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	26	41	51
Interest cost	101	76	69
Expected return on assets	(83)	(99)	(99)
Amortization of:
Prior service cost (credit)	(10)	(19)	(25)
Actuarial loss (gain)	(2)	12	27
Settlement and other charges	0	0	1
Net periodic benefit cost	$ 32	$ 11	$ 24