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Claim, Claim Adjustment Expense 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, 2021 USD ($) claim | Dec. 31, 2020 USD ($) | 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 ($) | Dec. 31, 2012 USD ($)
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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 | $ 4,776 |  |  |  |  |  |  |  |  |
IBNR | 1,016 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 497 | $ 497 | $ 499 | $ 493 | $ 484 | $ 493 | $ 498 | $ 508 | $ 469 | $ 464
IBNR | $ 4 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,755 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 530 | 531 | 545 | 535 | 525 | 513 | 500 | 479 | $ 462 |
IBNR | $ 11 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,565 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 524 | 527 | 529 | 535 | 530 | 537 | 489 | $ 450 |  |
IBNR | $ 8 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,800 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 501 | 510 | 488 | 494 | 510 | 499 | $ 433 |  |  |
IBNR | $ 28 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,170 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 510 | 508 | 499 | 485 | 487 | $ 427 |  |  |  |
IBNR | $ 24 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 16,085 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 455 | 460 | 458 | 449 | $ 412 |  |  |  |  |
IBNR | $ 41 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 15,197 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 448 | 431 | 429 | $ 404 |  |  |  |  |  |
IBNR | $ 60 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 14,997 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 458 | 445 | $ 430 |  |  |  |  |  |  |
IBNR | $ 156 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 13,804 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 476 | $ 477 |  |  |  |  |  |  |  |
IBNR | $ 347 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 9,935 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 377 |  |  |  |  |  |  |  |  |
IBNR | $ 337 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 6,761 |  |  |  |  |  |  |  |  |

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