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Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax $ 10 $ (11) $ 43 $ (55)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax (7) 4 (24) 24  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 3 (7) 19 (31)  
Cash Flow Hedges [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax 2   2   $ 0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax (1)   (1)    
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax 1   1    
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 1 0 1 0 $ 0
Pension and OPEB Plans [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax (10) (14) (30) (42)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 4 5 12 17  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (6) (9) (18) (25)  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (6) (9) (18) (25)  
Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax 18 3 71 (13)  
Reclassification for Available for Sale Securities, Tax (10) (1) (35) 7  
Reclassification for Available for Sale Securities, After-Tax 8 2 36 (6)  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 8 2 36 (6)  
Power [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax 10 (9) 36 (50)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax (6) 4 (21) 22  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 4 (5) 15 (28)  
Power [Member] | Cash Flow Hedges [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 0 0 0 0  
Power [Member] | Pension and OPEB Plans [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax (9) (12) (26) (36)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 4 5 11 15  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (5) (7) (15) (21)  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (5) (7) (15) (21)  
Power [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax 19 3 62 (14)  
Reclassification for Available for Sale Securities, Tax (10) (1) (32) 7  
Reclassification for Available for Sale Securities, After-Tax 9 2 30 (7)  
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 9 2 30 (7)  
Interest Expense [Member] | Cash Flow Hedges [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax 2   2    
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax (1)   (1)    
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax 1   1    
Operating Expense [Member] | Pension and OPEB Plans [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Amortization of Prior Service (Cost) Credit, Pre-Tax 3 3 7 9  
Amortization of Prior Service (Cost) Credit, Tax (1) (2) (3) (4)  
Amortization of Prior Service (Cost) Credit, After-Tax 2 1 4 5  
Amortization of Actuarial Loss, Pre-Tax (13) (17) (37) (51)  
Amortization of Actuarial Loss, Tax 5 7 15 21  
Amortization of Actuarial Loss, After-Tax (8) (10) (22) (30)  
Operating Expense [Member] | Power [Member] | Pension and OPEB Plans [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Amortization of Prior Service (Cost) Credit, Pre-Tax 2 3 6 8  
Amortization of Prior Service (Cost) Credit, Tax (1) (1) (3) (3)  
Amortization of Prior Service (Cost) Credit, After-Tax 1 2 3 5  
Amortization of Actuarial Loss, Pre-Tax (11) (15) (32) (44)  
Amortization of Actuarial Loss, Tax 5 6 14 18  
Amortization of Actuarial Loss, After-Tax (6) (9) (18) (26)  
Other Income [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax 29 13 99 41  
Reclassification for Available for Sale Securities, Tax (15) (6) (49) (20)  
Reclassification for Available for Sale Securities, After-Tax 14 7 50 21  
Other Income [Member] | Power [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax 29 12 86 37  
Reclassification for Available for Sale Securities, Tax (15) (5) (44) (18)  
Reclassification for Available for Sale Securities, After-Tax 14 7 42 19  
Other Expense [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax (6) (5) (19) (29)  
Reclassification for Available for Sale Securities, Tax 2 3 9 15  
Reclassification for Available for Sale Securities, After-Tax (4) (2) (10) (14)  
Other Expense [Member] | Power [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax (5) (4) (15) (26)  
Reclassification for Available for Sale Securities, Tax 2 2 7 13  
Reclassification for Available for Sale Securities, After-Tax (3) (2) (8) (13)  
Other-Than-Temporary Impairments [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax (5) (5) (9) (25)  
Reclassification for Available for Sale Securities, Tax 3 2 5 12  
Reclassification for Available for Sale Securities, After-Tax (2) (3) (4) (13)  
Other-Than-Temporary Impairments [Member] | Power [Member] | Available-for-Sale Securities [Member]          
Reclassification out of Accumulated Other Comprehensive Income [Line Items]          
Reclassification for Available for Sale Securities, Pre-Tax (5) (5) (9) (25)  
Reclassification for Available for Sale Securities, Tax 3 2 5 12  
Reclassification for Available for Sale Securities, After-Tax $ (2) $ (3) $ (4) $ (13)