Derivative Financial Instruments (Derivatives Not Designated as Hedging Instruments) (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	[1]	$ (1)	$ 0
Interest Rate Contract [Member] | Interest Charges [Member]
Derivative [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	[1]	$ (1)	$ 0

[1] Includes derivative activity amortized out of regulatory assets/liabilities. Amounts deferred into regulatory assets/liabilities have no associated effect in the Consolidated Statements of Comprehensive Income (Loss).