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Derivative Assets And Liabilities (Fair Value Of Derivative Instruments) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Asset Derivatives $ 321 $ 363
Liability Derivatives (560) (672)
Designated as Hedging Instrument [Member]    
Asset Derivatives 14 0
Liability Derivatives (2) (4)
Designated as Hedging Instrument [Member] | Commodity Derivatives (Margin Deposits) [Member]    
Asset Derivatives 14 0
Liability Derivatives (2) (4)
Not Designated as Hedging Instrument [Member]    
Asset Derivatives 307 363
Liability Derivatives (558) (668)
Not Designated as Hedging Instrument [Member] | Commodity Derivatives (Margin Deposits) [Member]    
Asset Derivatives 262 338
Liability Derivatives (281) (416)
Not Designated as Hedging Instrument [Member] | Commodity Derivatives [Member]    
Asset Derivatives 45 25
Liability Derivatives (58) (58)
Not Designated as Hedging Instrument [Member] | Interest Rate Derivatives [Member]    
Asset Derivatives 0 0
Liability Derivatives (219) (193)
Not Designated as Hedging Instrument [Member] | Embedded Derivatives [Member]    
Asset Derivatives 0 0
Liability Derivatives 0 (1)
Broker cleared derivative contracts [Member]    
Asset Derivatives 276 338
Liability Derivatives $ (283) $ (420)