| **Segment Information (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended |        |
|---------------------------------------------------------------------------------|----------------|--------|
||
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | $ 1,103        | $ 986  |   |
| Income (loss) before income taxes                                               | 217            | 112    |   |
| Identifiable assets                                                             | 17,037         |   | 16,548 |
| Property Casualty Insurance Segment                                             |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Identifiable assets                                                             | 2,403          |   | 2,395 |
| Commercial Lines                                                                |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 631            | 569    |   |
| Income (loss) before income taxes                                               | 58             | 34     |   |
| Commercial Lines | Commercial Casualty Segment                                  |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 204            | 181    |   |
| Commercial Lines | Commercial Property Segment                                  |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 147            | 131    |   |
| Commercial Lines | Commercial Auto Segment                                      |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 114            | 101    |   |
| Commercial Lines | Workers' Compensation                                        |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 88             | 81     |   |
| Commercial Lines | Specialty Packages                                           |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 39             | 38     |   |
| Commercial Lines | Surety and Executive Risk                                    |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 29             | 27     |   |
| Commercial Lines | Machinery and Equipment                                      |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 10             | 9      |   |
| Commercial Lines | Commercial Lines Insurance Premiums                          |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 631            | 568    |   |
| Commercial Lines | Other Fee Revenue                                            |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 0              | 1      |   |
| Personal Lines                                                                  |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Income (loss) before income taxes                                               | 20             | (22)   |   |
| Personal Lines | Personal Auto                                                  |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 107            | 98     |   |
| Personal Lines | Home Owner Segment                                             |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 96             | 84     |   |
| Personal Lines | Other Personal Lines                                           |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 28             | 27     |   |
| Personal Lines | Commercial Lines Insurance Premiums                            |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 231            | 209    |   |
| Excess And Surplus Lines Insurance                                              |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 27             | 21     |   |
| Income (loss) before income taxes                                               | 0              | (3)    |   |
| Life Insurance Segment                                                          |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 43             | 41     |   |
| Income (loss) before income taxes                                               | 7              | (3)    |   |
| Identifiable assets                                                             | 1,193          |   | 1,201 |
| Life Insurance Segment | Life Insurance Excluding Account Investment Fees       |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 42             | 41     |   |
| Life Insurance Segment | Seperate Account Investment Management Fees            |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 1              | 0      |   |
| Investments Segment                                                             |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 169            | 144    |   |
| Income (loss) before income taxes                                               | 148            | 123    |   |
| Identifiable assets                                                             | 13,099         |   | 12,599 |
| Investments Segment | Investment Income, Net                                    |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 128            | 131    |   |
| Investments Segment | Realized Investment Gains, Net                            |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 41             | 13     |   |
| Other                                                                           |           |   |   |
| **Segment Reporting Information [Line Items]**                                  |           |   |   |
| Revenues                                                                        | 2              | 2      |   |
| Income (loss) before income taxes                                               | (16)           | (17)   |   |
| Identifiable assets                                                             | $ 342          |   | $ 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Concepts (CON) -Number 6 -Paragraph 25 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.18) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(1)(i)) -URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 1(i) -Article 4
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5
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| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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