Financial Risk Management Activities (Schedule Of Derivative Instruments Designated As Cash Flow Hedges) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Pre-Tax Gain (Loss) attributed to Cash Flow Hedges Recognized in AOCI on Derivatives (Effective Portion)	$ 0	$ 3	$ 3
Amount of Pre-Tax Gain (Loss) Reclassified from AOCI into Income, Effective Portion	(3)	0	(20)
Power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Pre-Tax Gain (Loss) attributed to Cash Flow Hedges Recognized in AOCI on Derivatives (Effective Portion)	0	0	3
Amount of Pre-Tax Gain (Loss) Reclassified from AOCI into Income, Effective Portion	0	0	(20)
Operating Revenues [Member] | Energy-Related Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Pre-Tax Gain (Loss) attributed to Cash Flow Hedges Recognized in AOCI on Derivatives (Effective Portion)	0	0	3
Amount of Pre-Tax Gain (Loss) Reclassified from AOCI into Income, Effective Portion	0	0	(20)
Operating Revenues [Member] | Power [Member] | Energy-Related Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Pre-Tax Gain (Loss) attributed to Cash Flow Hedges Recognized in AOCI on Derivatives (Effective Portion)	0	0	3
Amount of Pre-Tax Gain (Loss) Reclassified from AOCI into Income, Effective Portion	0	0	(20)
Interest Expense [Member] | Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Pre-Tax Gain (Loss) attributed to Cash Flow Hedges Recognized in AOCI on Derivatives (Effective Portion)	0	3	0
Amount of Pre-Tax Gain (Loss) Reclassified from AOCI into Income, Effective Portion	$ (3)	$ 0	$ 0