| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions** | Mar. 31, 2018 | Dec. 31, 2017 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Debt Securities                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | $ 10,364      | $ 10,314      |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 243           | 409           |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 79            | 24            |
| Fair Value                                                                                                                                              | 10,528        | 10,699        |
| Debt Securities | Corporate                                                                                                                             |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 5,452         | 5,420         |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 155           | 246           |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 35            | 13            |
| Fair Value                                                                                                                                              | 5,572         | 5,653         |
| Debt Securities | States, municipalities and political subdivisions                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 4,304         | 4,316         |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 85            | 155           |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 35            | 6             |
| Fair Value                                                                                                                                              | 4,354         | 4,465         |
| Debt Securities | Commercial mortgage-backed                                                                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 285           | 280           |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 3             | 7             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 2             | 1             |
| Fair Value                                                                                                                                              | 286           | 286           |
| Debt Securities | Government-sponsored enterprises                                                                                                      |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 277           | 257           |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 0             | 1             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 6             | 4             |
| Fair Value                                                                                                                                              | 271           | 254           |
| Debt Securities | United States government                                                                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 36            | 31            |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 0             | 0             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 1             | 0             |
| Fair Value                                                                                                                                              | 35            | 31            |
| Debt Securities | Foreign government                                                                                                                    |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  | 10            | 10            |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            | 0             | 0             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            | 0             | 0             |
| Fair Value                                                                                                                                              | $ 10          | 10            |
| Equity securities                                                                                                                                       |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  |          | 3,094         |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            |          | 3,169         |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            |          | 14            |
| Fair Value                                                                                                                                              |          | 6,249         |
| Equity securities | Common equities                                                                                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  |          | 2,918         |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            |          | 3,135         |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            |          | 14            |
| Fair Value                                                                                                                                              |          | 6,039         |
| Equity securities | Nonredeemable preferred equities                                                                                                    |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  |          | 176           |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            |          | 34            |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            |          | 0             |
| Fair Value                                                                                                                                              |          | 210           |
| Fixed Maturities And Equity Securities                                                                                                                  |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                              |          |          |
| Cost or Amortized Cost                                                                                                                                  |          | 13,408        |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                            |          | 3,578         |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                            |          | 38            |
| Fair Value                                                                                                                                              |          | $ 16,948      |

| 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
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ 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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| 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.
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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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| X                                                                                                                                                                    |
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| X                                                                                                                                                                          |
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