Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Current income taxes, Federal	$ 72	$ (1)	$ 53
Current income taxes, State	10	6	12
Deferred income taxes, Federal	(13)	170	76
Deferred income taxes, State	13	23	11
Investment tax credits amortization	(1)	(1)	0
Total income tax expense	$ 81	$ 197	$ 152