Retirement Benefits - Net Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Defined Contribution Plan Disclosure [Line Items]
Service cost	$ 26	$ 26
Interest cost	113	117
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(135)	(138)
Prior service (credit) cost	(2)	(2)
Actuarial (gain) loss	23	9
Settlement charges	3	0
Non-service components of pension benefits & OPEB (credit) cost	2	(14)
Net periodic benefit cost	28	12
Pension Benefits
Defined Contribution Plan Disclosure [Line Items]
Service cost	22	22
Interest cost	95	99
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(124)	(127)
Prior service (credit) cost	0	0
Actuarial (gain) loss	25	12
Settlement charges	3	0
Non-service components of pension benefits & OPEB (credit) cost	(1)	(16)
Net periodic benefit cost	21	6
OPEB
Defined Contribution Plan Disclosure [Line Items]
Service cost	4	4
Interest cost	18	18
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(11)	(11)
Prior service (credit) cost	(2)	(2)
Actuarial (gain) loss	(2)	(3)
Settlement charges	0	0
Non-service components of pension benefits & OPEB (credit) cost	3	2
Net periodic benefit cost	7	6
Pension Plan and Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Net periodic benefit cost	$ 24	$ 24