| **Segment Information (Details) $ in Millions**                                      | 3 Months Ended | 12 Months Ended |
|--------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | $ 4,710 | $ 4,480 | $ 4,243 |
| Total revenues                                                                       | $ 1,312        | $ 1,402         | $ 1,371 | $ 1,364 | $ 1,263 | $ 1,278 | $ 1,316 | $ 1,285 | 5,449 | 5,142 | 4,945 |
| Other Income                                                                         |           |            |   |   |   |   |   |   | 5 | 7 | 8 |
| Fees and Commissions                                                                 |           |            |   |   |   |   |   |   | 15 | 13 | 12 |
| Net Investment Income                                                                |           |            |   |   |   |   |   |   | 595 | 572 | 549 |
| Total                                                                                |           |            |   |   |   |   |   |   | 124 | 70 | 133 |
| Income before income taxes                                                           | 128            | $ 253           | $ 166 | $ 265 | 216 | $ 243 | $ 248 | $ 174 | 812 | 881 | 721 |
| Identifiable assets                                                                  | 20,386         |            |   |   | 18,888 |   |   |   | $ 20,386 | 18,888 |   |
| Number of Industries Operated In | industry                                          |           |            |   |   |   |   |   |   | 2 |   |   |
| Number of Insurance Segments | segment                                               |           |            |   |   |   |   |   |   | 4 |   |   |
| Life Insurance Segment                                                               |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | $ 228 | 209 | 198 |
| Identifiable assets                                                                  | 1,366          |            |   |   | 1,325 |   |   |   | 1,366 | 1,325 |   |
| Investments Segment                                                                  |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Identifiable assets                                                                  | 15,569         |            |   |   | 14,485 |   |   |   | 15,569 | 14,485 |   |
| Consolidated Property and Casualty Insurance Entity                                  |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Identifiable assets                                                                  | 2,967          |            |   |   | 2,717 |   |   |   | 2,967 | 2,717 |   |
| Corporate and Other [Member]                                                         |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Identifiable assets                                                                  | $ 484          |            |   |   | $ 361 |   |   |   | 484 | 361 |   |
| Operating Segments | Commercial Insurance                                            |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 3,089 | 2,996 | 2,856 |
| Total revenues                                                                       |           |            |   |   |   |   |   |   | 3,094 | 3,000 | 2,860 |
| Fees and Commissions                                                                 |           |            |   |   |   |   |   |   | 5 | 4 | 4 |
| Income before income taxes                                                           |           |            |   |   |   |   |   |   | 184 | 345 | 146 |
| Operating Segments | Commercial Insurance | Commercial Casualty Line                 |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 1,050 | 1,010 | 938 |
| Operating Segments | Commercial Insurance | Commercial Property Line                 |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 867 | 815 | 728 |
| Operating Segments | Commercial Insurance | Commercial Auto Line                     |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 594 | 561 | 528 |
| Operating Segments | Commercial Insurance | Workers' Compensation Line               |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 354 | 367 | 370 |
| Operating Segments | Commercial Insurance | Other Commercial Lines                   |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 224 | 243 | 292 |
| Operating Segments | Personal Insurance                                              |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 1,161 | 1,097 | 1,041 |
| Total revenues                                                                       |           |            |   |   |   |   |   |   | 1,165 | 1,100 | 1,043 |
| Fees and Commissions                                                                 |           |            |   |   |   |   |   |   | 4 | 3 | 2 |
| Income before income taxes                                                           |           |            |   |   |   |   |   |   | (12) | (12) | 10 |
| Operating Segments | Personal Insurance | Personal Auto                              |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 543 | 506 | 476 |
| Operating Segments | Personal Insurance | Home Owner Line                            |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 486 | 463 | 443 |
| Operating Segments | Personal Insurance | Other Personal Lines                       |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 132 | 128 | 122 |
| Operating Segments | Excess and Surplus Lines Insurance                              |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 183 | 168 | 148 |
| Total revenues                                                                       |           |            |   |   |   |   |   |   | 184 | 169 | 148 |
| Fees and Commissions                                                                 |           |            |   |   |   |   |   |   | 1 | 1 | 0 |
| Income before income taxes                                                           |           |            |   |   |   |   |   |   | 62 | 51 | 30 |
| Operating Segments | Life Insurance Segment                                          |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 228 | 209 | 198 |
| Total revenues                                                                       |           |            |   |   |   |   |   |   | 233 | 214 | 204 |
| Fees and Commissions                                                                 |           |            |   |   |   |   |   |   | 5 | 5 | 6 |
| Income before income taxes                                                           |           |            |   |   |   |   |   |   | 1 | (2) | (5) |
| Operating Segments | Investments Segment                                             |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Total revenues                                                                       |           |            |   |   |   |   |   |   | 719 | 642 | 682 |
| Net Investment Income                                                                |           |            |   |   |   |   |   |   | 595 | 572 | 549 |
| Total                                                                                |           |            |   |   |   |   |   |   | 124 | 70 | 133 |
| Income before income taxes                                                           |           |            |   |   |   |   |   |   | 629 | 556 | 599 |
| Corporate, Non-Segment [Member]                                                      |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Total revenues                                                                       |           |            |   |   |   |   |   |   | 54 | 17 | 8 |
| Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member] |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Earned premiums                                                                      |           |            |   |   |   |   |   |   | 49 | 10 | 0 |
| Corporate, Non-Segment [Member] | Corporate and Other [Member]                       |           |            |   |   |   |   |   |   |   |   |   |
| **Segment Reporting Information**                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Other Income                                                                         |           |            |   |   |   |   |   |   | 5 | 7 | 8 |
| Income before income taxes                                                           |           |            |   |   |   |   |   |   | $ (52) | $ (57) | $ (59) |

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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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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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