Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) in AOCI	$ (7)	$ 23	$ (23)	$ 53
(Gain) Loss into Income	8	(7)	18	(16)
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	(1)	0	(1)	0	0
(Gain) Loss into Income	0	$ 0	0	$ 0	(2)
After-Tax Balance at End of Period	$ (1)		$ (1)		$ 0