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Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - Commercial $ 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 | $ 2,771 |  |  |  |  |  |  |  |  |
IBNR | 521 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 243 | $ 241 | $ 241 | $ 241 | $ 245 | $ 245 | $ 249 | $ 265 | $ 265 | $ 246
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 39,431 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 201 | 202 | 201 | 201 | 205 | 205 | 212 | 223 | $ 234 |
IBNR | $ 1 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 33,631 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 182 | 183 | 181 | 183 | 190 | 190 | 199 | $ 201 |  |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
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 | $ 200 | 195 | 190 | 186 | 186 | 186 | $ 198 |  |  |
IBNR | $ 7 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,452 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 239 | 232 | 221 | 200 | 198 | $ 199 |  |  |  |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
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 | $ 254 | 245 | 227 | 227 | $ 229 |  |  |  |  |
IBNR | $ 5 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 34,319 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 323 | 289 | 266 | $ 257 |  |  |  |  |  |
IBNR | $ 27 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 37,237 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 304 | 303 | $ 310 |  |  |  |  |  |  |
IBNR | $ 56 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 29,070 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 388 | $ 397 |  |  |  |  |  |  |  |
IBNR | $ 153 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 32,575 |  |  |  |  |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 437 |  |  |  |  |  |  |  |  |
IBNR | $ 263 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,229 |  |  |  |  |  |  |  |  |

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