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, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 526	$ 506	$ 447
Federal statutory income tax rates	21.00%	21.00%	21.00%
Income taxes, at statutory income tax rate	$ 110	$ 106	$ 94
State income tax, net of federal income tax	13	17	15
Excess deferred tax amortization	(26)	(26)	(25)
ITC amortization	(9)	(7)	(5)
AFUDC-equity	(9)	(6)	(2)
Tax credits	(3)	(8)	(1)
Other	4	6	1
Total income tax expense	$ 80	$ 82	$ 77
Income tax expense as a percent of income before income taxes	15.20%	16.20%	17.20%