| **Segment Information (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |         |
|---------------------------------------------------------------------------------|----------------|----------------|---------|
||
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | $ 1,152        | $ 1,035        | $ 3,359 | $ 3,041 |   |
| Net Investment Income                                                           | 133            | 132            | 392     | 395 |   |
| Realized gains on investments                                                   | 22             | 10             | 77      | 29 |   |
| Income (loss) before income taxes                                               | 182            | 152            | 547     | 293 |   |
| Identifiable assets                                                             | 17,339         |           | 17,339  |   | 16,548 |
| Property Casualty Insurance Segment                                             |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Identifiable assets                                                             | 2,479          |           | 2,479   |   | 2,395 |
| Commercial Lines                                                                |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 681            | 607            | 1,958   | 1,767 |   |
| Income (loss) before income taxes                                               | 43             | 60             | 135     | 74 |   |
| Commercial Lines | Commercial Casualty Segment                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 221            | 197            | 636     | 569 |   |
| Commercial Lines | Commercial Property Segment                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 161            | 138            | 460     | 403 |   |
| Commercial Lines | Commercial Auto Segment                                      |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 124            | 108            | 355     | 315 |   |
| Commercial Lines | Workers' Compensation                                        |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 95             | 89             | 270     | 255 |   |
| Commercial Lines | Specialty Packages                                           |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 38             | 37             | 114     | 112 |   |
| Commercial Lines | Surety and Executive Risk                                    |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 30             | 28             | 89      | 82 |   |
| Commercial Lines | Machinery and Equipment                                      |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 11             | 10             | 32      | 29 |   |
| Commercial Lines | Commercial Lines Insurance Premiums                          |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 680            | 607            | 1,956   | 1,765 |   |
| Commercial Lines | Other Fee Revenue                                            |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 1              | 0              | 2       | 2 |   |
| Personal Lines                                                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 244            | 220            | 713     | 644 |   |
| Income (loss) before income taxes                                               | 14             | (12)           | 33      | (89) |   |
| Personal Lines | Personal Auto                                                  |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 112            | 101            | 328     | 299 |   |
| Personal Lines | Home Owner Segment                                             |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 103            | 90             | 298     | 261 |   |
| Personal Lines | Other Personal Lines                                           |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 29             | 28             | 86      | 82 |   |
| Personal Lines | Commercial Lines Insurance Premiums                            |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 244            | 219            | 712     | 642 |   |
| Personal Lines | Other Fee Revenue                                              |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 0              | 1              | 1       | 2 |   |
| Excess And Surplus Lines Insurance                                              |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 30             | 25             | 85      | 68 |   |
| Income (loss) before income taxes                                               | 4              | (3)            | 5       | (8) |   |
| Life Insurance Segment                                                          |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 39             | 39             | 127     | 131 |   |
| Income (loss) before income taxes                                               | 0              | 0              | 10      | (2) |   |
| Identifiable assets                                                             | 1,263          |           | 1,263   |   | 1,201 |
| Life Insurance Segment | Life Insurance Excluding Account Investment Fees       |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 38             | 38             | 124     | 130 |   |
| Life Insurance Segment | Seperate Account Investment Management Fees            |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 1              | 1              | 3       | 1 |   |
| Investments Segment                                                             |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 155            | 142            | 469     | 424 |   |
| Income (loss) before income taxes                                               | 134            | 121            | 409     | 362 |   |
| Identifiable assets                                                             | 13,137         |           | 13,137  |   | 12,599 |
| Investments Segment | Investment Income, Net                                    |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Net Investment Income                                                           | 133            | 132            | 392     | 395 |   |
| Investments Segment | Realized Investment Gains, Net                            |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Realized gains on investments                                                   | 22             | 10             | 77      | 29 |   |
| Other                                                                           |           |           |    |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |           |    |   |   |
| Revenues                                                                        | 3              | 2              | 7       | 7 |   |
| Income (loss) before income taxes                                               | (13)           | (14)           | (45)    | (44) |   |
| Identifiable assets                                                             | $ 460          |           | $ 460   |   | $ 353 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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).
+ References
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| X                                                                                                                                                                        |
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