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Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - Commercial $ 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 | $ 2,582 |  |  |  |  |  |  |  |  |
IBNR | 433 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 295 | $ 296 | $ 297 | $ 299 | $ 299 | $ 307 | $ 303 | $ 299 | $ 289 | $ 275
IBNR | $ 1 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 46,288 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 241 | 241 | 241 | 245 | 245 | 249 | 265 | 265 | $ 246 |
IBNR | $ 2 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 39,430 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 202 | 201 | 201 | 205 | 205 | 212 | 223 | $ 234 |  |
IBNR | $ 2 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 33,628 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 183 | 181 | 183 | 190 | 190 | 199 | $ 201 |  |  |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,427 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 195 | 190 | 186 | 186 | 186 | $ 198 |  |  |  |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,449 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 232 | 221 | 200 | 198 | $ 199 |  |  |  |  |
IBNR | $ 5 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 30,940 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 245 | 227 | 227 | $ 229 |  |  |  |  |  |
IBNR | $ 8 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 34,292 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 289 | 266 | $ 257 |  |  |  |  |  |  |
IBNR | $ 31 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 37,142 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 303 | $ 310 |  |  |  |  |  |  |  |
IBNR | $ 107 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 28,837 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 397 |  |  |  |  |  |  |  |  |
IBNR | $ 271 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 27,182 |  |  |  |  |  |  |  |  |

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