Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) in AOCI	$ (6)		$ 25	$ (29)	$ 78
(Gain) Loss into Income	9		(3)	27	(19)
Cash Flow Hedges [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Pre-Tax Balance at Beginning of Period				0	3	$ 3
Gain (Loss) in AOCI				(2)		0
(Gain) Loss into Income				0		(3)
Pre-Tax Balance at End of Period	(2)			(2)		0
After-Tax Balance at Beginning of Period				0	2	2
Gain (Loss) in AOCI	0	$ (1)	0	(1)	0	0
(Gain) Loss into Income	0		$ (1)	0	$ (1)	(2)
After-Tax Balance at End of Period	$ (1)			$ (1)		$ 0