Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Current income taxes, Federal	$ 56	$ 72	$ (1)
Current income taxes, State	6	10	6
Deferred income taxes, Federal	7	(13)	170
Deferred income taxes, State	13	13	23
Investment tax credits amortization	(5)	(1)	(1)
Total income tax expense	$ 77	$ 81	$ 197