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Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Workers' Compensation - Commercial $ 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 | $ 4,232 |  |  |  |  |  |  |  |  |
IBNR | 1,216 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 324 | $ 334 | $ 353 | $ 372 | $ 382 | $ 394 | $ 408 | $ 406 | $ 431 | $ 422
IBNR | $ 45 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 31,916 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 293 | 308 | 331 | 355 | 366 | 382 | 396 | 405 | $ 426 |
IBNR | $ 45 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 32,000 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 383 | 398 | 399 | 402 | 400 | 421 | 432 | $ 440 |  |
IBNR | $ 65 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 33,156 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 399 | 404 | 415 | 415 | 428 | 440 | $ 450 |  |  |
IBNR | $ 65 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 34,914 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 416 | 419 | 436 | 437 | 449 | $ 452 |  |  |  |
IBNR | $ 67 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 34,377 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 393 | 414 | 446 | 466 | $ 477 |  |  |  |  |
IBNR | $ 107 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 29,481 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 421 | 432 | 454 | $ 468 |  |  |  |  |  |
IBNR | $ 116 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,126 |  |  |  |  |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 478 | 489 | $ 497 |  |  |  |  |  |  |
IBNR | $ 148 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 33,428 |  |  |  |  |  |  |  |  |
2023 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 551 | $ 555 |  |  |  |  |  |  |  |
IBNR | $ 233 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 36,822 |  |  |  |  |  |  |  |  |
2024 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 574 |  |  |  |  |  |  |  |  |
IBNR | $ 325 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 34,332 |  |  |  |  |  |  |  |  |

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