| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2013 | Dec. 31, 2012 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Debt Securities                                                                                                                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | $ 8,323       | $ 8,222       |
| Gross Unrealized Gains                                                                                                                                                         | 850           | 874           |
| Gross Unrealized Losses                                                                                                                                                        | 4             | 3             |
| Fair Value                                                                                                                                                                     | 9,169         | 9,093         |
| Debt Securities | States, Municipalities And Political Subdivisions                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 3,006         | 3,040         |
| Gross Unrealized Gains                                                                                                                                                         | 233           | 250           |
| Gross Unrealized Losses                                                                                                                                                        | 1             | 1             |
| Fair Value                                                                                                                                                                     | 3,238         | 3,289         |
| Debt Securities | Convertibles And Bonds With Warrants Attached                                                                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 27            | 31            |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 27            | 31            |
| Debt Securities | United States Government                                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 7             | 7             |
| Gross Unrealized Gains                                                                                                                                                         | 1             | 1             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 8             | 8             |
| Debt Securities | Government-Sponsored Enterprises                                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 168           | 164           |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 1             | 0             |
| Fair Value                                                                                                                                                                     | 167           | 164           |
| Debt Securities | Foreign Government                                                                                                                                           |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 13            | 3             |
| Gross Unrealized Gains                                                                                                                                                         | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 13            | 3             |
| Debt Securities | Commercial Mortgage Backed Securities                                                                                                                        |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 38            | 27            |
| Gross Unrealized Gains                                                                                                                                                         | 1             | 1             |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 0             |
| Fair Value                                                                                                                                                                     | 39            | 28            |
| Debt Securities | Corporate Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 5,064         | 4,950         |
| Gross Unrealized Gains                                                                                                                                                         | 615           | 622           |
| Gross Unrealized Losses                                                                                                                                                        | 2             | 2             |
| Fair Value                                                                                                                                                                     | 5,677         | 5,570         |
| Equity Securities                                                                                                                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 2,416         | 2,369         |
| Gross Unrealized Gains                                                                                                                                                         | 1,387         | 1,014         |
| Gross Unrealized Losses                                                                                                                                                        | 2             | 10            |
| Fair Value                                                                                                                                                                     | 3,801         | 3,373         |
| Equity Securities | Common Stock                                                                                                                                               |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 2,331         | 2,270         |
| Gross Unrealized Gains                                                                                                                                                         | 1,346         | 977           |
| Gross Unrealized Losses                                                                                                                                                        | 2             | 9             |
| Fair Value                                                                                                                                                                     | 3,675         | 3,238         |
| Equity Securities | Preferred Stock                                                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 85            | 99            |
| Gross Unrealized Gains                                                                                                                                                         | 41            | 37            |
| Gross Unrealized Losses                                                                                                                                                        | 0             | 1             |
| Fair Value                                                                                                                                                                     | 126           | 135           |
| Fixed Maturities And Equity Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |          |          |
| Cost or Amortized Cost                                                                                                                                                         | 10,739        | 10,591        |
| Gross Unrealized Gains                                                                                                                                                         | 2,237         | 1,888         |
| Gross Unrealized Losses                                                                                                                                                        | 6             | 13            |
| Fair Value                                                                                                                                                                     | $ 12,970      | $ 12,466      |

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| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
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| - Definition
This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 22 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27161-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 13, 19 -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
This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
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| X                                                                                                                                                                                 |
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