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, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 506	$ 447	$ 422
Federal statutory income tax rates	21.00%	21.00%	21.00%
Income taxes, at statutory income tax rate	$ 106	$ 94	$ 89
State income tax, net of federal income tax	17	15	19
Excess deferred tax amortization	(26)	(25)	(24)
ITC amortization	(7)	(5)	(1)
AFUDC-equity	(6)	(2)	(2)
Tax credits	(8)	(1)	(2)
Other	6	1	2
Total income tax expense	$ 82	$ 77	$ 81
Income tax expense as a percent of income before income taxes	16.20%	17.20%	19.20%