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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 ($) | Dec. 31, 2015 USD ($)
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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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