| **Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions** | Mar. 31, 2024 | Dec. 31, 2023 |
|-------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | $ 14,084      | $ 13,791      |
| Fair value of asset                                                                                                                       | 26,585        | 25,701        |
| Separate accounts                                                                                                                         | 927           | 925           |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                            | 78            | 67            |
| Corporate                                                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 7,599         | 7,452         |
| States, municipalities and political subdivisions                                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 4,668         | 4,703         |
| Government-sponsored enterprises                                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 1,421         | 1,224         |
| Asset-backed                                                                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 189           | 187           |
| United States government                                                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 178           | 200           |
| Foreign government                                                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 29            | 25            |
| Level 1                                                                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 178           | 200           |
| Fair value of asset                                                                                                                       | 11,459        | 10,908        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                            | 78            | 67            |
| Level 1 | Corporate                                                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 1 | States, municipalities and political subdivisions                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 1 | Government-sponsored enterprises                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 1 | Asset-backed                                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 1 | United States government                                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 178           | 200           |
| Level 1 | Foreign government                                                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 2                                                                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 13,906        | 13,591        |
| Fair value of asset                                                                                                                       | 15,126        | 14,793        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                            | 0             | 0             |
| Level 2 | Corporate                                                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 7,599         | 7,452         |
| Level 2 | States, municipalities and political subdivisions                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 4,668         | 4,703         |
| Level 2 | Government-sponsored enterprises                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 1,421         | 1,224         |
| Level 2 | Asset-backed                                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 189           | 187           |
| Level 2 | United States government                                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 2 | Foreign government                                                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 29            | 25            |
| Level 3                                                                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Fair value of asset                                                                                                                       | 0             | 0             |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                            | 0             | 0             |
| Level 3 | Corporate                                                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 3 | States, municipalities and political subdivisions                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 3 | Government-sponsored enterprises                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 3 | Asset-backed                                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 3 | United States government                                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Level 3 | Foreign government                                                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fixed maturities, at fair value                                                                                                           | 0             | 0             |
| Equity securities | Common equities                                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 11,203        | 10,641        |
| Equity securities | Nonredeemable preferred equities                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 354           | 348           |
| Equity securities | Level 1 | Common equities                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 11,203        | 10,641        |
| Equity securities | Level 1 | Nonredeemable preferred equities                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 0             | 0             |
| Equity securities | Level 2 | Common equities                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 0             | 0             |
| Equity securities | Level 2 | Nonredeemable preferred equities                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 354           | 348           |
| Equity securities | Level 3 | Common equities                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 0             | 0             |
| Equity securities | Level 3 | Nonredeemable preferred equities                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Fair value of asset                                                                                                                       | 0             | 0             |
| Separate accounts taxable fixed maturities                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Separate accounts                                                                                                                         | 866           | 854           |
| Separate accounts taxable fixed maturities | Level 1                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Separate accounts                                                                                                                         | 0             | 0             |
| Separate accounts taxable fixed maturities | Level 2                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Separate accounts                                                                                                                         | 866           | 854           |
| Separate accounts taxable fixed maturities | Level 3                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                                       |          |          |
| Separate accounts                                                                                                                         | $ 0           | $ 0           |

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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
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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.
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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.
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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.
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| - Definition
Amount of asset at fair value held for benefit of separate account policyholder.
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