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Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - 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 | $ 3,745 |  |  |  |  |  |  |  |  |
IBNR | 820 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 184 | $ 184 | $ 182 | $ 183 | $ 181 | $ 183 | $ 190 | $ 190 | $ 199 | $ 201
IBNR | $ 2 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,430 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 195 | 197 | 200 | 195 | 190 | 186 | 186 | 186 | $ 198 |
IBNR | $ 1 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,457 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 241 | 241 | 239 | 232 | 221 | 200 | 198 | $ 199 |  |
IBNR | $ 4 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,947 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 260 | 255 | 254 | 245 | 227 | 227 | $ 229 |  |  |
IBNR | $ 2 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 34,333 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 327 | 325 | 323 | 289 | 266 | $ 257 |  |  |  |
IBNR | $ 7 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 37,280 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 303 | 298 | 304 | 303 | $ 310 |  |  |  |  |
IBNR | $ 14 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 29,182 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 393 | 390 | 388 | $ 397 |  |  |  |  |  |
IBNR | $ 51 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 33,028 |  |  |  |  |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 496 | 465 | $ 437 |  |  |  |  |  |  |
IBNR | $ 90 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 37,230 |  |  |  |  |  |  |  |  |
2023 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 620 | $ 554 |  |  |  |  |  |  |  |
IBNR | $ 202 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 42,345 |  |  |  |  |  |  |  |  |
2024 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 726 |  |  |  |  |  |  |  |  |
IBNR | $ 447 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 40,718 |  |  |  |  |  |  |  |  |

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