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Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ in Millions | Dec. 31, 2022 USD ($) claim | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
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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 | $ 4,624 |  |  |  |  |  |  |  |  |
IBNR | 988 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 530 | $ 530 | $ 531 | $ 545 | $ 535 | $ 525 | $ 513 | $ 500 | $ 479 | $ 462
IBNR | $ 5 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,587 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 527 | 524 | 527 | 529 | 535 | 530 | 537 | 489 | $ 450 |
IBNR | $ 12 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,818 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 498 | 501 | 510 | 488 | 494 | 510 | 499 | $ 433 |  |
IBNR | $ 22 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,203 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 508 | 510 | 508 | 499 | 485 | 487 | $ 427 |  |  |
IBNR | $ 12 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 16,136 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 460 | 455 | 460 | 458 | 449 | $ 412 |  |  |  |
IBNR | $ 30 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 15,288 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 470 | 448 | 431 | 429 | $ 404 |  |  |  |  |
IBNR | $ 31 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 15,163 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 471 | 458 | 445 | $ 430 |  |  |  |  |  |
IBNR | $ 101 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 14,189 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 455 | 476 | $ 477 |  |  |  |  |  |  |
IBNR | $ 226 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 10,679 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 376 | $ 377 |  |  |  |  |  |  |  |
IBNR | $ 259 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 8,801 |  |  |  |  |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 329 |  |  |  |  |  |  |  |  |
IBNR | $ 290 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 6,717 |  |  |  |  |  |  |  |  |

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