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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 422	$ 513	$ 438
Federal statutory income tax rates	21.00%	35.00%	35.00%
Income taxes, at statutory income tax rate	$ 89	$ 180	$ 153
State income tax, net of federal income tax	19	19	15
Excess deferred tax amortization	(24)	0	0
AFUDC-equity	(2)	(1)	(8)
Tax credits	(2)	(3)	(7)
Other	1	2	(1)
Total income tax expense	$ 81	$ 197	$ 152
Income tax expense as a percent of income from continuing operations, before income taxes	19.20%	38.40%	34.80%