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Derivative Instruments (Income Statement Disclosure) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]        
Gains (losses) on commodity contracts, recorded as regulatory assets and or liabilities on the balance sheet due to regulatory treatment $ 178 $ 23 $ 70 $ (63)
Cash Flow Hedging [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Gains (Losses) Recognized in Other Comprehensive Income (Loss)   66   (11)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (34)   (55)
Cash Flow Hedging [Member] | Interest Rate Contract [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Gains (Losses) Recognized in Other Comprehensive Income (Loss)   73   4
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (19)   (38)
Cash Flow Hedging [Member] | Currency Swap [Member] | Interest Expense [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (3)   (6)
Cash Flow Hedging [Member] | Foreign Exchange Contract [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Gains (Losses) Recognized in Other Comprehensive Income (Loss)   (7)   (15)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   (15)   (17)
Not Designated as Hedging Instrument [Member]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (349) 202 (196) 428
Not Designated as Hedging Instrument [Member] | 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 (92) 178 238 415
Not Designated as Hedging Instrument [Member] | 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 (4) 0 (2) 2
Not Designated as Hedging Instrument [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 (278) 24 (457) 11
Not Designated as Hedging Instrument [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 52 0 81 0
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Gains (Losses) Included In Other - Net [Member]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 1 0 3 0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Contract [Member] | Gain Loss Included In Interest Expense Member        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]        
Interest Expense (25) 0 (53) 0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Gain Loss Included In Interest Expense Member        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]        
Interest Expense $ (3) $ 0 $ (6) $ 0