Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Gain (Loss) Recognized in AOCI, After-Tax	$ 77	$ (3)	$ (30)
Less: Gain Reclassified to Income, After-Tax	(43)	35	18
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivative [Line Items]
Balance as of Beginning of Year	3	0
Gain (Loss) Recognized in AOCI, Pre-Tax	0	3
Less: Gain Reclassified into Income, Pre-Tax	(3)	0	(20)
Balance as of End of Year	0	3	0
Balance as of Beginning of Year	2	0
Gain (Loss) Recognized in AOCI, After-Tax	0	2	2
Less: Gain Reclassified to Income, After-Tax	(2)	0	(12)
Balance as of End of Year	$ 0	$ 2	$ 0