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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Derivative [Line Items]      
Gain (Loss) Recognized in AOCI, After-Tax $ (30) $ (135) $ 299
Less: Gain Reclassified to Income, After-Tax 18 (53) (6)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]      
Derivative [Line Items]      
Balance as of Beginning of Year 17 (4)  
Gain (Loss) Recognized in AOCI, Pre-Tax 3 12  
Less: Gain Reclassified into Income, Pre-Tax (20) 9 (12)
Balance as of End of Year 0 17 (4)
Balance as of Beginning of Year 10 (2)  
Gain (Loss) Recognized in AOCI, After-Tax 2 7 (2)
Less: Gain Reclassified to Income, After-Tax (12) 5 (7)
Balance as of End of Year $ 0 $ 10 $ (2)