| **Segment Information (Detail) - USD ($) $ in Millions**           | 3 Months Ended | 9 Months Ended |          |
|--------------------------------------------------------------------|----------------|----------------|----------|
||
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Assets                                                             | $ 26,370       |           | $ 26,370 |   | $ 25,408 |
| Earned premiums                                                    | 1,522          | $ 1,446        | 4,460    | $ 4,163 |   |
| Fee revenues                                                       | 2              | 4              | 8        | 11 |   |
| Other revenues                                                     | 3              | 3              | 8        | 7 |   |
| Investment income, net of expenses                                 | 167            | 161            | 498      | 478 |   |
| Investment gains and losses, net                                   | 533            | 86             | (132)    | 1,113 |   |
| Total revenues                                                     | 2,227          | 1,700          | 4,842    | 5,772 |   |
| Income Before Income Taxes                                         | 614            | 294            | 183      | 1,691 |   |
| Consolidated Property and Casualty Insurance Entity                |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Assets                                                             | 3,796          |           | 3,796    |   | 3,437 |
| Earned premiums                                                    | 1,450          | 1,376          | 4,242    | 3,960 |   |
| Life Insurance Segment                                             |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Assets                                                             | 1,619          |           | 1,619    |   | 1,516 |
| Investments Segment                                                |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Assets                                                             | 20,155         |           | 20,155   |   | 19,583 |
| Corporate, Non-Segment                                             |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Assets                                                             | 800            |           | 800      |   | $ 872 |
| Other revenues                                                     | 3              | 3              | 8        | 7 |   |
| Total revenues                                                     | 139            | 119            | 324      | 252 |   |
| Income Before Income Taxes                                         | (55)           | (4)            | (78)     | (16) |   |
| Corporate, Non-Segment | Reinsurance assumed and other non segment |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 136            | 116            | 316      | 245 |   |
| Operating Segments | Commercial Lines                              |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 865            | 834            | 2,598    | 2,467 |   |
| Fee revenues                                                       | 1              | 1              | 3        | 3 |   |
| Total revenues                                                     | 866            | 835            | 2,601    | 2,470 |   |
| Income Before Income Taxes                                         | (20)           | 56             | (32)     | 144 |   |
| Operating Segments | Personal Lines                                |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 367            | 354            | 1,090    | 1,046 |   |
| Fee revenues                                                       | 1              | 1              | 3        | 3 |   |
| Total revenues                                                     | 368            | 355            | 1,093    | 1,049 |   |
| Income Before Income Taxes                                         | (2)            | 3              | (24)     | 4 |   |
| Operating Segments | Excess and Surplus Lines Insurance            |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 82             | 72             | 238      | 202 |   |
| Fee revenues                                                       | 0              | 1              | 1        | 2 |   |
| Total revenues                                                     | 82             | 73             | 239      | 204 |   |
| Income Before Income Taxes                                         | 11             | 12             | 19       | 40 |   |
| Operating Segments | Life Insurance Segment                        |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 72             | 70             | 218      | 203 |   |
| Fee revenues                                                       | 0              | 1              | 1        | 3 |   |
| Total revenues                                                     | 72             | 71             | 219      | 206 |   |
| Income Before Income Taxes                                         | 6              | 5              | 9        | 2 |   |
| Operating Segments | Investments Segment                           |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Investment income, net of expenses                                 | 167            | 161            | 498      | 478 |   |
| Investment gains and losses, net                                   | 533            | 86             | (132)    | 1,113 |   |
| Total revenues                                                     | 700            | 247            | 366      | 1,591 |   |
| Income Before Income Taxes                                         | 674            | 222            | 289      | 1,517 |   |
| Commercial Casualty | Operating Segments | Commercial Lines        |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 290            | 277            | 868      | 822 |   |
| Commercial Property | Operating Segments | Commercial Lines        |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 252            | 241            | 755      | 709 |   |
| Commercial Auto | Operating Segments | Commercial Lines            |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 189            | 179            | 563      | 524 |   |
| Workers' Compensation | Operating Segments | Commercial Lines      |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 64             | 73             | 207      | 224 |   |
| Other Commercial Lines | Operating Segments | Commercial Lines     |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 70             | 64             | 205      | 188 |   |
| Personal Auto | Operating Segments | Personal Lines                |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 154            | 156            | 462      | 466 |   |
| Home Owner Segment | Operating Segments | Personal Lines           |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | 165            | 154            | 487      | 450 |   |
| Other Personal Lines | Operating Segments | Personal Lines         |           |           |     |   |   |
| **Segment Reporting Information**                                  |           |           |     |   |   |
| Earned premiums                                                    | $ 48           | $ 44           | $ 141    | $ 130 |   |

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| - Definition
Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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| - Definition
Amount of realized and unrealized gain (loss) on investment.
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| - Definition
Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.
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| - Definition
Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.
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| - Definition
Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
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Amount of revenue and income classified as other.
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| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
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| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
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| X                                                                                                                                           |
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| X                                                                                                                                             |
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