| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions** | Sep. 30, 2016 | Dec. 31, 2015 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Debt Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | $ 9,657       | $ 9,324       |
| Gross Unrealized Gains                                                                                                                                  | 615           | 431           |
| Gross Unrealized Losses                                                                                                                                 | 15            | 105           |
| Fair Value                                                                                                                                              | 10,257        | 9,650         |
| Debt Securities | Corporate                                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 5,621         | 5,294         |
| Gross Unrealized Gains                                                                                                                                  | 375           | 255           |
| Gross Unrealized Losses                                                                                                                                 | 14            | 96            |
| Fair Value                                                                                                                                              | 5,982         | 5,453         |
| Debt Securities | States, municipalities and political subdivisions                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 3,619         | 3,440         |
| Gross Unrealized Gains                                                                                                                                  | 222           | 172           |
| Gross Unrealized Losses                                                                                                                                 | 1             | 1             |
| Fair Value                                                                                                                                              | 3,840         | 3,611         |
| Debt Securities | Commercial mortgage-backed                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 290           | 287           |
| Gross Unrealized Gains                                                                                                                                  | 18            | 4             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 2             |
| Fair Value                                                                                                                                              | 308           | 289           |
| Debt Securities | Government-sponsored enterprises                                                                                                      |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 108           | 284           |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 6             |
| Fair Value                                                                                                                                              | 108           | 278           |
| 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                                                                                                                                  | 5             | 5             |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 0             |
| Fair Value                                                                                                                                              | 5             | 5             |
| Debt Securities | United States government                                                                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 4             | 4             |
| Gross Unrealized Gains                                                                                                                                  | 0             | 0             |
| Gross Unrealized Losses                                                                                                                                 | 0             | 0             |
| Fair Value                                                                                                                                              | 4             | 4             |
| Equity Securities                                                                                                                                       |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 3,169         | 2,938         |
| Gross Unrealized Gains                                                                                                                                  | 2,196         | 1,819         |
| Gross Unrealized Losses                                                                                                                                 | 61            | 51            |
| Fair Value                                                                                                                                              | 5,304         | 4,706         |
| Equity Securities | Common equities                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 2,982         | 2,749         |
| Gross Unrealized Gains                                                                                                                                  | 2,156         | 1,787         |
| Gross Unrealized Losses                                                                                                                                 | 61            | 51            |
| Fair Value                                                                                                                                              | 5,077         | 4,485         |
| Equity Securities | Nonredeemable preferred equities                                                                                                    |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 187           | 189           |
| Gross Unrealized Gains                                                                                                                                  | 40            | 32            |
| Gross Unrealized Losses                                                                                                                                 | 0             | 0             |
| Fair Value                                                                                                                                              | 227           | 221           |
| Fixed Maturities And Equity Securities                                                                                                                  |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 12,826        | 12,262        |
| Gross Unrealized Gains                                                                                                                                  | 2,811         | 2,250         |
| Gross Unrealized Losses                                                                                                                                 | 76            | 156           |
| Fair Value                                                                                                                                              | $ 15,561      | $ 14,356      |

| 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
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+ 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                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| 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                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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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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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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