| **Claims Development - Commercial Auto (Details) reported_claim in Thousands, $ in Millions** | Dec. 31, 2022 USD ($) reported_claim | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) |
|-----------------------------------------------------------------------------------------------|--------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | $ 3,312                              |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                             | 7,967                                |                  |                  |                  |                  |
| Commercial Auto Line | Commercial Insurance                                                   |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | 2,332                                |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                           | 1,581                                |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                        | 40                                   |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                             | 791                                  |                  |                  |                  |                  |
| 2018 | Commercial Auto Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | 444                                  | $ 440                 | $ 442                 | $ 442                 | $ 453                 |
| Total of incurred but not reported liabilities plus expected development on reported losses   | $ 9                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                         | 49                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                           | $ 411                                | 378                   | 337                   | 266                   | $ 184                 |
| 2019 | Commercial Auto Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | 469                                  | 453                   | 451                   | 452                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses   | $ 18                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                         | 46                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                           | $ 395                                | 333                   | 268                   | $ 183                 |                  |
| 2020 | Commercial Auto Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | 384                                  | 391                   | 424                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses   | $ 37                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                         | 36                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                           | $ 280                                | 214                   | $ 154                 |                  |                  |
| 2021 | Commercial Auto Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | 477                                  | 470                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses   | $ 91                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                         | 39                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                           | $ 278                                | $ 179                 |                  |                  |                  |
| 2022 | Commercial Auto Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                        |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                 | 558                                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses   | $ 181                                |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                         | 37                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                           | $ 217                                |                  |                  |                  |                  |

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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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| - Definition
Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
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| - Definition
Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
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Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
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| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.
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Cumulative number of reported claims for short-duration insurance contracts.
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