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Derivative Instruments (Income Statement Disclosure) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   $ (22)
Effect of Cash Flow Hedges on Results of Operations [Abstract]    
Gains (Losses) Recognized in Other Comprehensive Income (Loss)   (78)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 152 226
Gains (losses) on commodity contracts, recorded as regulatory assets and or liabilities on the balance sheet due to regulatory treatment (108) (86)
Commodity Contract [Member] | Gains (losses) included in operating revenues [Member]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 330 237
Commodity Contract [Member] | Gains (losses) included in fuel, purchased power and interchange [Member]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 2 2
Interest Rate Contract [Member]    
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (20)
Effect of Cash Flow Hedges on Results of Operations [Abstract]    
Gains (Losses) Recognized in Other Comprehensive Income (Loss)   (70)
Interest Rate Contract [Member] | Gain Loss Included In Interest Expense Member    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (179) (13)
Foreign Exchange Contract [Member]    
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (2)
Effect of Cash Flow Hedges on Results of Operations [Abstract]    
Gains (Losses) Recognized in Other Comprehensive Income (Loss)   (8)
Foreign Exchange Contract [Member] | Gain Loss Included In Interest Expense Member    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 30 0
Cash Flow Hedging [Member] | Currency Swap [Member] | Interest Expense [Member]    
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (3)
Foreign Exchange Contract [Member] | Foreign Exchange Contract [Member] | Gain Loss Included In Interest Expense Member    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (3) 0
Interest Rate Contract [Member] | Interest Rate Contract [Member] | Gain Loss Included In Interest Expense Member    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ (28) $ 0