Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Current income taxes, Federal	$ 35	$ 56	$ 72
Current income taxes, State	(7)	6	10
Deferred income taxes, Federal	32	7	(13)
Deferred income taxes, State	29	13	13
Investment tax credits amortization	(7)	(5)	(1)
Total income tax expense	$ 82	$ 77	$ 81