Income Taxes - Schedule of Income Taxes Calculated on Income before Income Taxes and Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 536	$ 553	$ 661
Federal statutory income tax rates	21.00%	21.00%	21.00%
Income taxes, at statutory income tax rate	$ 113	$ 116	$ 139
State income tax, net of federal income tax	23	23	27
Excess deferred tax amortization	(25)	(25)	(25)
ITC amortization	(4)	(8)	(6)
AFUDC-equity	(3)	(4)	(7)
Production tax credits	(30)	(15)	(6)
Other tax credits	(7)	(4)	(3)
Other	1	4	2
Total income tax expense	$ 68	$ 87	$ 121
Income tax expense as a percent of income before income taxes	12.70%	15.70%	18.30%