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Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Mar. 31, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax $ (19)   $ (12) $ (44) $ (4)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax 9   5 20 1  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (10)   (7) (24) (3)  
Cash Flow Hedges [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Energy-Related Contracts, Pre-Tax 0 $ 0 0   17 $ 20
Energy-Related Contracts, Tax     0   (7)  
Energy-Related Contracts, After-Tax     0   10  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0 $ 0 0 0 10 $ 12
Pension and OPEB Plans [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax (14)   (14) (28) (28)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 6   6 12 12  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (8)   (8) (16) (16)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (8)   (8) (16) (16)  
Available-for-Sale Securities [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Reclassification for Available for Sale Securities, Pre-Tax (5)   2 (16) 7  
Reclassification for Available for Sale Securities, Tax 3   (1) 8 (4)  
Reclassification for Available for Sale Securities, After-Tax (2)   1 (8) 3  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (2)   1 (8) 3  
Power [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax (18)   (12) (41) (2)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax 8   5 18 0  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (10)   (7) (23) (2)  
Power [Member] | Cash Flow Hedges [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Energy-Related Contracts, Pre-Tax     0   17  
Energy-Related Contracts, Tax     0   (7)  
Energy-Related Contracts, After-Tax     0   10  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0   0 0 10  
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 (12)   (12) (24) (24)  
Reclassification Adjustment from AOCI, Pension and OPEB, Tax 5   5 10 10  
Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax (7)   (7) (14) (14)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (7)   (7) (14) (14)  
Power [Member] | Available-for-Sale Securities [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Reclassification for Available for Sale Securities, Pre-Tax (6)   0 (17) 5  
Reclassification for Available for Sale Securities, Tax 3   0 8 (3)  
Reclassification for Available for Sale Securities, After-Tax (3)   0 (9) 2  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax (3)   0 (9) 2  
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 6 6  
Amortization of Prior Service (Cost) Credit, Tax (1)   (1) (2) (2)  
Amortization of Prior Service (Cost) Credit, After-Tax 2   2 4 4  
Amortization of Actuarial Loss, Pre-Tax (17)   (17) (34) (34)  
Amortization of Actuarial Loss, Tax 7   7 14 14  
Amortization of Actuarial Loss, After-Tax (10)   (10) (20) (20)  
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 5 6  
Amortization of Prior Service (Cost) Credit, Tax (1)   (1) (2) (2)  
Amortization of Prior Service (Cost) Credit, After-Tax 1   2 3 4  
Amortization of Actuarial Loss, Pre-Tax (14)   (15) (29) (30)  
Amortization of Actuarial Loss, Tax 6   6 12 12  
Amortization of Actuarial Loss, After-Tax (8)   (9) (17) (18)  
Operating Revenues [Member] | Cash Flow Hedges [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Energy-Related Contracts, Pre-Tax     0   17  
Energy-Related Contracts, Tax     0   (7)  
Energy-Related Contracts, After-Tax     0   10  
Operating Revenues [Member] | Power [Member] | Cash Flow Hedges [Member]            
Reclassification out of Accumulated Other Comprehensive Income [Line Items]            
Energy-Related Contracts, Pre-Tax     0   17  
Energy-Related Contracts, Tax     0   (7)  
Energy-Related Contracts, After-Tax     0   10  
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 12   16 28 35  
Reclassification for Available for Sale Securities, Tax (6)   (8) (14) (18)  
Reclassification for Available for Sale Securities, After-Tax 6   8 14 17  
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 10   14 25 33  
Reclassification for Available for Sale Securities, Tax (5)   (7) (13) (17)  
Reclassification for Available for Sale Securities, After-Tax 5   7 12 16  
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 (7)   (4) (24) (13)  
Reclassification for Available for Sale Securities, Tax 4   2 12 7  
Reclassification for Available for Sale Securities, After-Tax (3)   (2) (12) (6)  
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 (6)   (4) (22) (13)  
Reclassification for Available for Sale Securities, Tax 3   2 11 7  
Reclassification for Available for Sale Securities, After-Tax (3)   (2) (11) (6)  
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 (10)   (10) (20) (15)  
Reclassification for Available for Sale Securities, Tax 5   5 10 7  
Reclassification for Available for Sale Securities, After-Tax (5)   (5) (10) (8)  
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 (10)   (10) (20) (15)  
Reclassification for Available for Sale Securities, Tax 5   5 10 7  
Reclassification for Available for Sale Securities, After-Tax $ (5)   $ (5) $ (10) $ (8)