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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 513	$ 438	$ 442
Federal statutory income tax rates	35.00%	35.00%	35.00%
Income taxes, at statutory income tax rate	$ 180	$ 153	$ 155
State income tax, net of federal income tax	19	15	15
AFUDC-equity	(1)	(8)	(6)
Tax credits	(3)	(7)	0
Other	2	(1)	2
Total income tax expense	$ 197	$ 152	$ 166
Income tax expense as a percent of income from continuing operations, before income taxes	38.40%	34.80%	37.50%