| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions** | Jun. 30, 2015 | Dec. 31, 2014 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Debt Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | $ 9,224       | $ 8,871       |
| Gross Unrealized Gains                                                                                                                                  | 493           | 607           |
| Gross Unrealized Losses                                                                                                                                 | 42            | 18            |
| Fair Value                                                                                                                                              | 9,675         | 9,460         |
| Debt Securities | Corporate                                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 5,243         | 5,117         |
| Gross Unrealized Gains                                                                                                                                  | 350           | 420           |
| Gross Unrealized Losses                                                                                                                                 | 16            | 11            |
| Fair Value                                                                                                                                              | 5,577         | 5,526         |
| Debt Securities | States, municipalities and political subdivisions                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 3,426         | 3,267         |
| Gross Unrealized Gains                                                                                                                                  | 136           | 178           |
| Gross Unrealized Losses                                                                                                                                 | 15            | 2             |
| Fair Value                                                                                                                                              | 3,547         | 3,443         |
| Debt Securities | Commercial mortgage-backed                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 278           | 250           |
| Gross Unrealized Gains                                                                                                                                  | 7             | 9             |
| Gross Unrealized Losses                                                                                                                                 | 2             | 0             |
| Fair Value                                                                                                                                              | 283           | 259           |
| Debt Securities | Government-sponsored enterprises                                                                                                      |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 253           | 213           |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 9             | 5             |
| Fair Value                                                                                                                                              | 244           | 208           |
| Debt Securities | Foreign government                                                                                                                    |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 10            | 10            |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 0             |
| Fair Value                                                                                                                                              | 10            | 10            |
| Debt Securities | Convertibles and bonds with warrants attached                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 7             | 7             |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 0             |
| Fair Value                                                                                                                                              | 7             | 7             |
| Debt Securities | United States government                                                                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 7             | 7             |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 0             |
| Fair Value                                                                                                                                              | 7             | 7             |
| Equity Securities                                                                                                                                       |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 2,828         | 2,728         |
| Gross Unrealized Gains                                                                                                                                  | 1,915         | 2,134         |
| Gross Unrealized Losses                                                                                                                                 | 9             | 4             |
| Fair Value                                                                                                                                              | 4,734         | 4,858         |
| Equity Securities | Common equities                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 2,628         | 2,583         |
| Gross Unrealized Gains                                                                                                                                  | 1,882         | 2,099         |
| Gross Unrealized Losses                                                                                                                                 | 8             | 3             |
| Fair Value                                                                                                                                              | 4,502         | 4,679         |
| Equity Securities | Nonredeemable preferred equities                                                                                                    |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 200           | 145           |
| Gross Unrealized Gains                                                                                                                                  | 33            | 35            |
| Gross Unrealized Losses                                                                                                                                 | 1             | 1             |
| Fair Value                                                                                                                                              | 232           | 179           |
| Fixed Maturities And Equity Securities                                                                                                                  |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 12,052        | 11,599        |
| Gross Unrealized Gains                                                                                                                                  | 2,408         | 2,741         |
| Gross Unrealized Losses                                                                                                                                 | 51            | 22            |
| Fair Value                                                                                                                                              | $ 14,409      | $ 14,318      |

| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateBondSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
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| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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