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Financial Risk Management Activities (Schedule Of Derivative Instruments Not Designated As Hedging Instruments And Impact On Results Of Operations) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Pre-Tax Gain (Loss) Recognized in Income on Derivatives $ 55 $ 222 $ 404
Operating Revenues [Member] | Energy-Related Contracts [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Pre-Tax Gain (Loss) Recognized in Income on Derivatives 72 230 412
Energy Costs [Member] | Energy-Related Contracts [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Pre-Tax Gain (Loss) Recognized in Income on Derivatives $ (17) $ (8) $ (8)