| **Segment Information (Detail) - USD ($) $ in Millions**                             | 3 Months Ended | 6 Months Ended |         |
|--------------------------------------------------------------------------------------|----------------|----------------|---------|
||
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | $ 1,173        | $ 1,111        | $ 2,327 | $ 2,205 |   |
| Fee revenues                                                                         | 3              | 3              | 6       | 6 |   |
| Investment income, net of expenses                                                   | 149            | 140            | 294     | 279 |   |
| Realized investment gains, net                                                       | 44             | 60             | 105     | 107 |   |
| Other                                                                                | 2              | 2              | 3       | 4 |   |
| Total revenues                                                                       | 1,371          | 1,316          | 2,735   | 2,601 |   |
| Income (loss) before income taxes                                                    | 166            | 248            | 431     | 422 |   |
| Identifiable assets                                                                  | 20,151         |           | 20,151  |   | $ 18,888 |
| Consolidated Property and Casualty Insurance Entity                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Identifiable assets                                                                  | 2,888          |           | 2,888   |   | 2,717 |
| Life Insurance Segment                                                               |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Identifiable assets                                                                  | 1,422          |           | 1,422   |   | 1,325 |
| Investments Segment                                                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Identifiable assets                                                                  | 15,506         |           | 15,506  |   | 14,485 |
| Corporate, Non-Segment [Member]                                                      |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Other                                                                                | 2              | 2              | 3       | 4 |   |
| Total revenues                                                                       | 12             | 2              | 23      | 4 |   |
| Income (loss) before income taxes                                                    | (17)           | (14)           | (30)    | (29) |   |
| Identifiable assets                                                                  | 335            |           | 335     |   | $ 361 |
| Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member] |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 10             | 0              | 20      | 0 |   |
| Operating Segments | Commercial Lines                                                |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 771            | 745            | 1,531   | 1,478 |   |
| Fee revenues                                                                         | 1              | 1              | 2       | 2 |   |
| Total revenues                                                                       | 772            | 746            | 1,533   | 1,480 |   |
| Income (loss) before income taxes                                                    | 26             | 97             | 76      | 123 |   |
| Operating Segments | Personal Lines                                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 288            | 272            | 571     | 540 |   |
| Fee revenues                                                                         | 1              | 0              | 2       | 1 |   |
| Total revenues                                                                       | 289            | 272            | 573     | 541 |   |
| Income (loss) before income taxes                                                    | (20)           | (25)           | 8       | (28) |   |
| Operating Segments | Excess and Surplus Lines Insurance                              |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 45             | 42             | 88      | 82 |   |
| Fee revenues                                                                         | 0              | 1              | 0       | 1 |   |
| Total revenues                                                                       | 45             | 43             | 88      | 83 |   |
| Income (loss) before income taxes                                                    | 5              | 11             | 22      | 15 |   |
| Operating Segments | Life Insurance Segment                                          |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 59             | 52             | 117     | 105 |   |
| Fee revenues                                                                         | 1              | 1              | 2       | 2 |   |
| Total revenues                                                                       | 60             | 53             | 119     | 107 |   |
| Income (loss) before income taxes                                                    | 1              | 1              | 0       | (2) |   |
| Operating Segments | Investments Segment                                             |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Investment income, net of expenses                                                   | 149            | 140            | 294     | 279 |   |
| Realized investment gains, net                                                       | 44             | 60             | 105     | 107 |   |
| Total revenues                                                                       | 193            | 200            | 399     | 386 |   |
| Income (loss) before income taxes                                                    | 171            | 178            | 355     | 343 |   |
| Commercial Casualty | Operating Segments | Commercial Lines                          |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 263            | 252            | 520     | 496 |   |
| Commercial Property | Operating Segments | Commercial Lines                          |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 215            | 203            | 429     | 399 |   |
| Commercial Auto | Operating Segments | Commercial Lines                              |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 147            | 139            | 291     | 275 |   |
| Workers' Compensation | Operating Segments | Commercial Lines                        |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 89             | 90             | 178     | 183 |   |
| Other Commercial Lines | Operating Segments | Commercial Lines                       |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 57             | 61             | 113     | 125 |   |
| Personal Auto | Operating Segments | Personal Lines                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 135            | 125            | 266     | 248 |   |
| Home Owner Segment | Operating Segments | Personal Lines                             |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | 121            | 114            | 240     | 228 |   |
| Other Personal Lines | Operating Segments | Personal Lines                           |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                       |           |           |    |   |   |
| Earned premiums                                                                      | $ 32           | $ 33           | $ 65    | $ 64 |   |

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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 fees and commissions from financial services and banking activities and correspondent clearing. Includes fees from depositor accounts, credit cards, merchant discounts, and fiduciary and trust activities, guarantee fees, investment advisory, management, and administrative fees, transfer agent fees, and insurance commissions.
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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
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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| - Definition
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
The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.
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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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