Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Amortization of:
Non-service net periodic benefit cost	$ (18)	$ 16	$ (18)	$ 48
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	39	58	116	177
Interest cost	145	110	434	330
Expected return on assets	(189)	(205)	(566)	(619)
Amortization of:
Prior service cost (credit)	1	1	2	3
Actuarial loss	41	73	125	222
Settlement charges	18	0	18	0
Non-service net periodic benefit cost	55	37	129	113
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	10	18	30
Interest cost	25	19	76	57
Expected return on assets	(21)	(25)	(63)	(75)
Amortization of:
Prior service cost (credit)	(2)	(5)	(7)	(15)
Actuarial loss	0	4	(1)	12
Settlement charges	0	0	0	0
Non-service net periodic benefit cost	$ 8	$ 3	$ 23	$ 9