Other Income (Expense), Net (Components of Other Income (Expense), Net) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Statement Of Income Captions [Line Items]
Operating revenue from contracts with customers	$ 1	$ 1	$ 4	$ 4
Other income	11	5	17	135
Other expense	(23)	(10)	(48)	(22)
Allowance for equity funds used during construction	(2)	3	0	7
Other income (expense), net	$ (13)	$ (1)	$ (27)	124
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