| **Income Taxes - Differences Between Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Income Tax Disclosure [Abstract]**                                                                                            |           |           |   |   |
| Tax at statutory rate                                                                                                           | $ 130          | $ 46           | $ 175 | $ 187 |
| **Increase (decrease) resulting from:**                                                                                         |           |           |   |   |
| Tax-exempt income from municipal bonds                                                                                          | (5)            | (9)            | (15) | (27) |
| Dividend received exclusion                                                                                                     | (3)            | (8)            | (11) | (25) |
| Impact of tax reform                                                                                                            | (50)           | 0              | (50) | 0 |
| Other                                                                                                                           | (7)            | (2)            | (6) | (5) |
| Total provision for income taxes                                                                                                | $ 65           | $ 27           | $ 93 | $ 130 |
| Tax at statutory rate                                                                                                           | 21.00%         | 35.00%         | 21.00% | 35.00% |
| **Increase (decrease) resulting from:**                                                                                         |           |           |   |   |
| Tax-exempt income from municipal bonds                                                                                          | (0.80%)        | (7.00%)        | (1.80%) | (5.10%) |
| Dividend received exclusion                                                                                                     | (0.50%)        | (6.20%)        | (1.30%) | (4.70%) |
| Impact of tax reform                                                                                                            | (8.10%)        | 0.00%          | (6.00%) | 0.00% |
| Other                                                                                                                           | (1.10%)        | (0.90%)        | (0.70%) | (0.80%) |
| Provision for income taxes                                                                                                      | 10.50%         | 20.90%         | 11.20% | 24.40% |

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| - Definition
Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).
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Percentage 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 deduction for dividend.
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Percentage 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
Percentage 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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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
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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 deduction for dividend.
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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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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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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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