Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Taxes [Line Items]
Federal statutory tax rate	21.00%	21.00%	21.00%
Regulatory liability due to obligation to transfer tax rate reduction expense benefit	$ 57,000,000	$ 23,000,000
Provision for income taxes	68,000,000	87,000,000	$ 121,000,000
General business credit	$ 361,000,000
Deferred tax general business credits expiration date	2027-2044
Investment tax credits	$ 270,000,000
Investment tax credit expiration date	2034 and 2044
Uncertain tax positions	$ 10,000,000	10,000,000	9,000,000	$ 6,000,000
Pre-tax charges	0	0	0
Penalties	0	0	0
Decrease in current income tax expense	(13,000,000)		$ (59,000,000)
Federal [Member] | Research and Development Tax Credits [Member]
Income Taxes [Line Items]
Uncertain tax positions	$ 10,000,000	$ 10,000,000