| **Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ in Millions** | Dec. 31, 2024 USD ($) claim | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 4,244                     |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | 916                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | 494                         | $ 494                 | $ 498                 | $ 501                 | $ 510                 | $ 488                 | $ 494                 | $ 510                 | $ 499                 | $ 433                 |
| IBNR                                                                                                                                                                                                                                       | $ 11                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 18,228                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 513                       | 514                   | 508                   | 510                   | 508                   | 499                   | 485                   | 487                   | $ 427                 |                  |
| IBNR                                                                                                                                                                                                                                       | $ 16                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 16,195                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 463                       | 456                   | 460                   | 455                   | 460                   | 458                   | 449                   | $ 412                 |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 18                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 15,384                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2018                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 499                       | 495                   | 470                   | 448                   | 431                   | 429                   | $ 404                 |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 28                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 15,331                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2019                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 481                       | 469                   | 471                   | 458                   | 445                   | $ 430                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 43                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 14,515                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2020                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 419                       | 447                   | 455                   | 476                   | $ 477                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 100                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 11,289                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2021                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 349                       | 374                   | 376                   | $ 377                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 117                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 9,935                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2022                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 333                       | 329                   | $ 329                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 143                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 9,965                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2023                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 350                       | $ 340                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 162                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 10,424                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2024                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 343                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 278                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 8,561                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ References
No definition available.
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| - Definition
Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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| - Definition
Cumulative number of reported claims for short-duration insurance contracts.
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