| **Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions** | Mar. 31, 2018 | Dec. 31, 2017 |
|------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | $ 10,528      | $ 10,699      |
| Equity securities, at fair value                                                                                                         | 6,086         | 6,249         |
| Separate accounts                                                                                                                        | 803           | 806           |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                           | 34            | 31            |
| Fair value of asset                                                                                                                      | 17,441        | 17,774        |
| Debt Securities                                                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 10,528        | 10,699        |
| Debt Securities | Corporate                                                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 5,572         | 5,653         |
| Debt Securities | States, municipalities and political subdivisions                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 4,354         | 4,465         |
| Debt Securities | Commercial mortgage-backed                                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 286           | 286           |
| Debt Securities | Government-sponsored enterprises                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 271           | 254           |
| Debt Securities | United States government                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 35            | 31            |
| Debt Securities | Foreign government                                                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 10            | 10            |
| Equity securities | Common equities                                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 5,893         | 6,039         |
| Equity securities | Nonredeemable preferred equities                                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 193           | 210           |
| Taxable Fixed Maturities                                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Separate accounts                                                                                                                        | 793           | 795           |
| Quoted prices in active markets for identical assets (Level 1)                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                           | 34            | 31            |
| Fair value of asset                                                                                                                      | 5,962         | 6,101         |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 35            | 31            |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Corporate                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | States, municipalities and political subdivisions     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Commercial mortgage-backed                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Government-sponsored enterprises                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | United States government                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 35            | 31            |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Foreign government                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Equity securities | Common equities                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 5,893         | 6,039         |
| Quoted prices in active markets for identical assets (Level 1) | Equity securities | Nonredeemable preferred equities                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Taxable Fixed Maturities                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Separate accounts                                                                                                                        | 0             | 0             |
| Significant other observable inputs (Level 2)                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                           | 0             | 0             |
| Fair value of asset                                                                                                                      | 11,474        | 11,667        |
| Significant other observable inputs (Level 2) | Debt Securities                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 10,488        | 10,662        |
| Significant other observable inputs (Level 2) | Debt Securities | Corporate                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 5,571         | 5,652         |
| Significant other observable inputs (Level 2) | Debt Securities | States, municipalities and political subdivisions                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 4,350         | 4,460         |
| Significant other observable inputs (Level 2) | Debt Securities | Commercial mortgage-backed                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 286           | 286           |
| Significant other observable inputs (Level 2) | Debt Securities | Government-sponsored enterprises                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 271           | 254           |
| Significant other observable inputs (Level 2) | Debt Securities | United States government                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Significant other observable inputs (Level 2) | Debt Securities | Foreign government                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 10            | 10            |
| Significant other observable inputs (Level 2) | Equity securities | Common equities                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 0             | 0             |
| Significant other observable inputs (Level 2) | Equity securities | Nonredeemable preferred equities                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 193           | 210           |
| Significant other observable inputs (Level 2) | Taxable Fixed Maturities                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Separate accounts                                                                                                                        | 793           | 795           |
| Significant unobservable inputs (Level 3)                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                           | 0             | 0             |
| Fair value of asset                                                                                                                      | 5             | 6             |
| Significant unobservable inputs (Level 3) | Debt Securities                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 5             | 6             |
| Significant unobservable inputs (Level 3) | Debt Securities | Corporate                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 1             | 1             |
| Significant unobservable inputs (Level 3) | Debt Securities | States, municipalities and political subdivisions                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 4             | 5             |
| Significant unobservable inputs (Level 3) | Debt Securities | Commercial mortgage-backed                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | Government-sponsored enterprises                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | United States government                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Significant unobservable inputs (Level 3) | Debt Securities | Foreign government                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Fixed maturities, at fair value                                                                                                          | 0             | 0             |
| Significant unobservable inputs (Level 3) | Equity securities | Common equities                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 0             | 0             |
| Significant unobservable inputs (Level 3) | Equity securities | Nonredeemable preferred equities                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Equity securities, at fair value                                                                                                         | 0             | 0             |
| Significant unobservable inputs (Level 3) | Taxable Fixed Maturities                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                         |          |          |
| Separate accounts                                                                                                                        | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of debt securities categorized neither as held-to-maturity nor trading.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of equity securities categorized neither as held-to-maturity nor as trading.
+ References
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| - Definition
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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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.
+ References
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| - Definition
Value of the other investment not readily marketable determined by management based upon fair value methods, including pricing of similar securities and valuation techniques, that was used to record the investment for financial reporting purposes.
+ References
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| - Definition
The fair value of the assets held by the Entity for the benefit of separate account policyholders.
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