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Derivative Instruments (Income Statement Disclosure) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 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 $ (35) $ (141) $ 35 $ (204)
Cash Flow Hedging [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Losses recognized in OCI   (152)   (162)
Gains (losses) reclassified from AOCI to net income   (11)   (66)
Cash Flow Hedging [Member] | Interest Rate Contract [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Losses recognized in OCI   (151)   (146)
Gains (losses) reclassified from AOCI to net income   (18)   (56)
Cash Flow Hedging [Member] | Currency Swap [Member] | Interest Expense [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Gains (losses) reclassified from AOCI to net income   (3)   (9)
Cash Flow Hedging [Member] | Foreign Exchange Contract [Member]        
Effect of Cash Flow Hedges on Results of Operations [Abstract]        
Losses recognized in OCI   (1)   (16)
Gains (losses) reclassified from AOCI to net income   7   (10)
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 202 388 0 816
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 264 397 502 812
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 1 3 (1) 5
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 (58) (12) (515) (1)
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 15 0 96 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 (2) 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 (18) 0 (71) 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 $ (9) $ 0