| **Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | $ 9,169       | $ 9,093       |
| Equity securities, at fair value                                                                                                                                | 3,801         | 3,373         |
| Separate accounts                                                                                                                                               | 726           | 734           |
| Fair value of asset                                                                                                                                             | 13,686        | 13,164        |
| Top Hat Savings Plan                                                                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Other investments                                                                                                                                               | 11            | 9             |
| Debt Securities                                                                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 9,169         | 9,093         |
| Debt Securities | States, Municipalities And Political Subdivisions                                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 3,238         | 3,289         |
| Debt Securities | Convertibles And Bonds With Warrants Attached                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 27            | 31            |
| Debt Securities | United States Government                                                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 8             | 8             |
| Debt Securities | Government-Sponsored Enterprises                                                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 167           | 164           |
| Debt Securities | Foreign Government                                                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 13            | 3             |
| Debt Securities | Commercial Mortgage Backed Securities                                                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 39            | 28            |
| Debt Securities | Corporate Securities                                                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 5,677         | 5,570         |
| Equity Securities | Common Stock                                                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 3,675         | 3,238         |
| Equity Securities | Preferred Stock                                                                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 126           | 135           |
| Taxable Fixed Maturities                                                                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Separate accounts                                                                                                                                               | 705           | 689           |
| Quoted prices in active markets for identical assets (Level 1)                                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fair value of asset                                                                                                                                             | 3,694         | 3,255         |
| Quoted prices in active markets for identical assets (Level 1) | Top Hat Savings Plan                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Other investments                                                                                                                                               | 11            | 9             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 8             | 8             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | States, Municipalities And Political Subdivisions                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Convertibles And Bonds With Warrants Attached                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | United States Government                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 8             | 8             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Government-Sponsored Enterprises                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Foreign Government                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Commercial Mortgage Backed Securities                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Corporate Securities                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Equity Securities | Common Stock                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 3,675         | 3,238         |
| Quoted prices in active markets for identical assets (Level 1) | Equity Securities | Preferred Stock                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Taxable Fixed Maturities                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Separate accounts                                                                                                                                               | 0             | 0             |
| Significant other observable inputs (Level 2)                                                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fair value of asset                                                                                                                                             | 9,986         | 9,904         |
| Significant other observable inputs (Level 2) | Top Hat Savings Plan                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Other investments                                                                                                                                               | 0             | 0             |
| Significant other observable inputs (Level 2) | Debt Securities                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 9,157         | 9,081         |
| Significant other observable inputs (Level 2) | Debt Securities | States, Municipalities And Political Subdivisions                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 3,237         | 3,288         |
| Significant other observable inputs (Level 2) | Debt Securities | Convertibles And Bonds With Warrants Attached                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 27            | 31            |
| Significant other observable inputs (Level 2) | Debt Securities | United States Government                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Significant other observable inputs (Level 2) | Debt Securities | Government-Sponsored Enterprises                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 167           | 164           |
| Significant other observable inputs (Level 2) | Debt Securities | Foreign Government                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 13            | 3             |
| Significant other observable inputs (Level 2) | Debt Securities | Commercial Mortgage Backed Securities                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 39            | 28            |
| Significant other observable inputs (Level 2) | Debt Securities | Corporate Securities                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 5,674         | 5,567         |
| Significant other observable inputs (Level 2) | Equity Securities | Common Stock                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 0             | 0             |
| Significant other observable inputs (Level 2) | Equity Securities | Preferred Stock                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 124           | 134           |
| Significant other observable inputs (Level 2) | Taxable Fixed Maturities                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Separate accounts                                                                                                                                               | 705           | 689           |
| Significant unobservable inputs (Level 3)                                                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fair value of asset                                                                                                                                             | 6             | 5             |
| Significant unobservable inputs (Level 3) | Top Hat Savings Plan                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Other investments                                                                                                                                               | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities                                                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 4             | 4             |
| Significant unobservable inputs (Level 3) | Debt Securities | States, Municipalities And Political Subdivisions                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 1             | 1             |
| Significant unobservable inputs (Level 3) | Debt Securities | Convertibles And Bonds With Warrants Attached                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | United States Government                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | Government-Sponsored Enterprises                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | Foreign Government                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | Commercial Mortgage Backed Securities                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | Corporate Securities                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Fixed maturities, at fair value                                                                                                                                 | 3             | 3             |
| Significant unobservable inputs (Level 3) | Equity Securities | Common Stock                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 0             | 0             |
| Significant unobservable inputs (Level 3) | Equity Securities | Preferred Stock                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Equity securities, at fair value                                                                                                                                | 2             | 1             |
| Significant unobservable inputs (Level 3) | Taxable Fixed Maturities                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |          |          |
| Separate accounts                                                                                                                                               | $ 0           | $ 0           |

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| - Definition
Amount of debt securities categorized neither as held-to-maturity nor trading.
+ References
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| - Definition
Amount of equity securities categorized neither as held-to-maturity nor as trading.
+ References
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| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method.
+ References
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| - Definition
Value of the other investment not readily marketable determined by management based upon fair value methods, including pricing of similar securities and valuation techniques, that was used to record the investment for financial reporting purposes.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 820 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7519002&loc=d3e45360-110995Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-BRD -Paragraph 6-14 -IssueDate 2006-05-01 -Chapter 7 -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.
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| - Definition
The fair value of the assets held by the Entity for the benefit of separate account policyholders.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 11 -Article 7Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 03-1 -Paragraph 10-18 -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 03 -Paragraph 10 -Article 9Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 53, 54 -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 80 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6484091&loc=d3e19268-158472Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -Glossary Separate Account -URI http://asc.fasb.org/extlink&oid=6524887
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