| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions** | Mar. 31, 2024 | Dec. 31, 2023 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | $ 2,258       | $ 1,384       |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 23            | 18            |
| 12 months or more, Fair Value                                                                                                                                                   | 7,799         | 7,912         |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 691           | 669           |
| Total Fair Value                                                                                                                                                                | 10,057        | 9,296         |
| Total Unrealized Losses                                                                                                                                                         | 714           | 687           |
| Corporate                                                                                                                                                                       |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | 642           | 379           |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 13            | 13            |
| 12 months or more, Fair Value                                                                                                                                                   | 5,488         | 5,560         |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 450           | 441           |
| Total Fair Value                                                                                                                                                                | 6,130         | 5,939         |
| Total Unrealized Losses                                                                                                                                                         | 463           | 454           |
| States, municipalities and political subdivisions                                                                                                                               |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | 1,099         | 313           |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 9             | 2             |
| 12 months or more, Fair Value                                                                                                                                                   | 1,900         | 1,932         |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 223           | 206           |
| Total Fair Value                                                                                                                                                                | 2,999         | 2,245         |
| Total Unrealized Losses                                                                                                                                                         | 232           | 208           |
| Government-sponsored enterprises                                                                                                                                                |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | 396           | 652           |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 1             | 3             |
| 12 months or more, Fair Value                                                                                                                                                   | 168           | 113           |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 3             | 3             |
| Total Fair Value                                                                                                                                                                | 564           | 765           |
| Total Unrealized Losses                                                                                                                                                         | 4             | 6             |
| Asset-backed                                                                                                                                                                    |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | 30            | 5             |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 0             | 0             |
| 12 months or more, Fair Value                                                                                                                                                   | 133           | 172           |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 11            | 16            |
| Total Fair Value                                                                                                                                                                | 163           | 177           |
| Total Unrealized Losses                                                                                                                                                         | 11            | 16            |
| United States government                                                                                                                                                        |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | 74            | 32            |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 0             | 0             |
| 12 months or more, Fair Value                                                                                                                                                   | 105           | 129           |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 4             | 3             |
| Total Fair Value                                                                                                                                                                | 179           | 161           |
| Total Unrealized Losses                                                                                                                                                         | 4             | 3             |
| Foreign government                                                                                                                                                              |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                       |          |          |
| Less than 12 months Fair Value                                                                                                                                                  | 17            | 3             |
| Less than 12 Months, Unrealized Loss                                                                                                                                            | 0             | 0             |
| 12 months or more, Fair Value                                                                                                                                                   | 5             | 6             |
| 12 months or longer, Unrealized Loss                                                                                                                                            | 0             | 0             |
| Total Fair Value                                                                                                                                                                | 22            | 9             |
| Total Unrealized Losses                                                                                                                                                         | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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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
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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| - Definition
Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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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), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.
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