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, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 447	$ 422	$ 513
Federal statutory income tax rates	21.00%	21.00%	35.00%
Income taxes, at statutory income tax rate	$ 94	$ 89	$ 180
State income tax, net of federal income tax	15	19	19
Excess deferred tax amortization	(25)	(24)	0
ITC amortization	(5)	(1)	(1)
AFUDC-equity	(2)	(2)	(1)
Tax credits	(1)	(2)	(3)
Other	1	2	3
Total income tax expense	$ 77	$ 81	$ 197
Income tax expense as a percent of income before income taxes	17.20%	19.20%	38.40%