Other Income (Expense), Net (Components of Other Income (Expense), Net) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Statement Of Income Captions [Line Items]
Operating revenue from contracts with customers	$ 2	$ 2	$ 3	$ 3
Other income	2	4	6	130
Other expense	(15)	(5)	(25)	(12)
Allowance for equity funds used during construction	2	1	2	4
Other income (expense), net	$ (9)	$ 2	$ (14)	125
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member]
Condensed Statement Of Income Captions [Line Items]
Gains from settlement of interest rate derivatives				115
Other Income [Member] | Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member]
Condensed Statement Of Income Captions [Line Items]
Gains from settlement of interest rate derivatives				$ 115