Accumulated Other Comprehensive Income (Loss), Net of Tax (Reclassifications of AOCI) (Details) - USD ($)
$ in Millions |
3 Months Ended |
12 Months Ended |
Mar. 31, 2018 |
Mar. 31, 2017 |
Dec. 31, 2017 |
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax |
$ (15)
|
$ 18
|
|
| Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax |
5
|
(9)
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
(10)
|
9
|
|
| Cash Flow Hedges [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
0
|
0
|
$ 2
|
| Pension and OPEB Plans [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Reclassification Adjustment from AOCI, Pension and OPEB, Pre-Tax |
(11)
|
(10)
|
|
| Reclassification Adjustment from AOCI, Pension and OPEB, Tax |
3
|
4
|
|
| Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax |
(8)
|
(6)
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
(8)
|
(6)
|
|
| Available-for-Sale Securities [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Reclassification for Available for Sale Securities, Pre-Tax |
(4)
|
28
|
|
| Reclassification for Available for Sale Securities, Tax |
2
|
(13)
|
|
| Reclassification for Available for Sale Securities, After-Tax |
(2)
|
15
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
(2)
|
15
|
|
| Power [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax |
(13)
|
10
|
|
| Reclassification from Accumulated Other Comprehensive Income, Current Period, Tax |
5
|
(6)
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
(8)
|
4
|
|
| 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
|
|
| 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)
|
(9)
|
|
| Reclassification Adjustment from AOCI, Pension and OPEB, Tax |
3
|
4
|
|
| Reclassification Adjustment from AOCI, Pension and OPEB, After-Tax |
(6)
|
(5)
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
(6)
|
(5)
|
|
| Power [Member] | Available-for-Sale Securities [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Reclassification for Available for Sale Securities, Pre-Tax |
(4)
|
19
|
|
| Reclassification for Available for Sale Securities, Tax |
2
|
(10)
|
|
| Reclassification for Available for Sale Securities, After-Tax |
(2)
|
9
|
|
| Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) |
(2)
|
9
|
|
| Non-Operating Pension and OPEB Credits (Costs) [Member] | Pension and OPEB Plans [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Amortization of Prior Service (Cost) Credit, Pre-Tax |
1
|
2
|
|
| Amortization of Prior Service (Cost) Credit, Tax |
0
|
(1)
|
|
| Amortization of Prior Service (Cost) Credit, After-Tax |
1
|
1
|
|
| Amortization of Actuarial Loss, Pre-Tax |
(12)
|
(12)
|
|
| Amortization of Actuarial Loss, Tax |
3
|
5
|
|
| Amortization of Actuarial Loss, After-Tax |
(9)
|
(7)
|
|
| Non-Operating Pension and OPEB Credits (Costs) [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 |
1
|
2
|
|
| Amortization of Prior Service (Cost) Credit, Tax |
0
|
(1)
|
|
| Amortization of Prior Service (Cost) Credit, After-Tax |
1
|
1
|
|
| Amortization of Actuarial Loss, Pre-Tax |
(10)
|
(11)
|
|
| Amortization of Actuarial Loss, Tax |
3
|
5
|
|
| Amortization of Actuarial Loss, After-Tax |
(7)
|
(6)
|
|
| Net Gains (Losses) on Trust Investments [Member] | Available-for-Sale Securities [Member] |
|
|
|
| Reclassification out of Accumulated Other Comprehensive Income [Line Items] |
|
|
|
| Reclassification for Available for Sale Securities, Pre-Tax |
(4)
|
28
|
|
| Reclassification for Available for Sale Securities, Tax |
2
|
(13)
|
|
| Reclassification for Available for Sale Securities, After-Tax |
(2)
|
15
|
|
| Net Gains (Losses) on Trust Investments [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 |
(4)
|
19
|
|
| Reclassification for Available for Sale Securities, Tax |
2
|
(10)
|
|
| Reclassification for Available for Sale Securities, After-Tax |
$ (2)
|
$ 9
|
|