| **Income Taxes (Reconciliation between the Company's federal income tax (expense) benefit at statutory rates and the recorded income tax (expense) benefit) (Details) - USD ($) $ in Millions** | 3 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Effective Income Tax Rate Reconciliation, Amount [Abstract]**                                                                                                                                 |           |            |   |   |
| Income tax expense at statutory rates                                                                                                                                                           |           | $ (459)         | $ (398) | $ (192) |
| Tax benefit from tax exempt income                                                                                                                                                              |           | 131             | 124 | 123 |
| Foreign taxes and credits                                                                                                                                                                       |           | 3               | 3 | 9 |
| Net deferred tax asset remeasurement                                                                                                                                                            | $ (83)         | (83)            | 0 | 0 |
| Other tax expense                                                                                                                                                                               |           | (3)             | (7) | (10) |
| Income tax expense                                                                                                                                                                              |           | $ (411)         | $ (278) | $ (70) |

| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Effective Income Tax Rate Reconciliation, Effect of Tax Cuts and Jobs Act of 2017, Amount
+ References
No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to foreign income tax expense (benefit).
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.
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| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.
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| - Definition
Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to income (loss) exempt from income taxes.
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