Derivative Financial Instruments (Derivatives Not Designated as Hedging Instruments) (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative [Line Items]
Increase (Decrease) in Derivatives Subject to Regulatory Treatment	[1]	$ (2)	$ 0	$ (3)	$ 65
Amount of gain (loss) recognized in income on derivatives	[2]	0	(1)	0	114
Other Income [Member]
Derivative [Line Items]
Amount of gain (loss) recognized in income on derivatives	[2]	0	0	0	115
Interest Rate Contract [Member] | Interest Charges [Member]
Derivative [Line Items]
Amount of gain (loss) recognized in income on derivatives	[2]	$ 0	$ (1)	$ 0	$ (1)

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

[2] 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).