Retirement Benefits - Gross Accumulated Other Comprehensive Loss and Regulatory Assets (Liabilities) not Recognized as Components of Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ 21	$ 19
Actuarial loss (gain)	3,948	3,611
Total	3,969	3,630
Benefits included in accumulated other comprehensive income	972	873
Benefits included in regulatory assets (liabilities)	2,997	2,757
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	(45)	(55)
Actuarial loss (gain)	(101)	(133)
Total	(146)	(188)
Benefits included in accumulated other comprehensive income	(17)	(21)
Benefits included in regulatory assets (liabilities)	$ (129)	$ (167)