Pension, Other Postretirement Benefits (OPEB) and Savings Plans - Schedule of Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Settlement Charge Resulting from Pension Lift-Out	$ 6	$ 332
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost (included in O&M Expense)			$ 94	$ 90	$ 142
Interest Cost			225	259	167
Expected Return on Plan Assets			(321)	(361)	(484)
Amortization of Net Prior Service Credit			0	0	0
Amortization of Net Actuarial Loss			71	83	60
Settlement Charge Resulting from Pension Lift-Out			0	338	0
Non-Service Components of Pension and OPEB (Credits) Costs			(25)	319	(257)
Total Net Benefit (Credits) Costs			69	409	(115)
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost (included in O&M Expense)			3	3	6
Interest Cost			37	41	26
Expected Return on Plan Assets			(34)	(33)	(42)
Amortization of Net Prior Service Credit			2	(52)	(129)
Amortization of Net Actuarial Loss			(2)	(2)	15
Settlement Charge Resulting from Pension Lift-Out			0	0	0
Non-Service Components of Pension and OPEB (Credits) Costs			3	(46)	(130)
Total Net Benefit (Credits) Costs			$ 6	$ (43)	$ (124)