| **Claims Development - Workers Compensation (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 ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) |
|----------------------------------------------------------------------------------------------------|--------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **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                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Workers' Compensation Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 1,865                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | 1,215                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                             | 311                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                  | 961                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 204                                  | $ 204                 | $ 201                 | $ 202                 | $ 205                 | $ 208                 | $ 212                 | $ 221                 | $ 246                 | $ 264                 |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 16                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 179                                | 177                   | 174                   | 170                   | 168                   | 164                   | 157                   | 144                   | 119                   | $ 61                  |
| 2014 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 202                                  | 202                   | 197                   | 198                   | 201                   | 203                   | 214                   | 233                   | 261                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 16                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 19                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 171                                | 168                   | 165                   | 162                   | 157                   | 148                   | 134                   | 110                   | $ 56                  |                  |
| 2015 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 171                                  | 173                   | 173                   | 179                   | 195                   | 208                   | 220                   | 246                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 23                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 17                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 139                                | 139                   | 137                   | 134                   | 129                   | 115                   | 93                    | $ 47                  |                  |                  |
| 2016 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 183                                  | 183                   | 183                   | 188                   | 206                   | 218                   | 230                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 24                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 16                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 148                                | 146                   | 141                   | 131                   | 119                   | 97                    | $ 46                  |                  |                  |                  |
| 2017 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 167                                  | 172                   | 183                   | 190                   | 208                   | 218                   |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 27                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 15                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 122                                | 119                   | 114                   | 106                   | 88                    | $ 45                  |                  |                  |                  |                  |
| 2018 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 179                                  | 186                   | 199                   | 207                   | 222                   |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 28                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 15                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 133                                | 127                   | 115                   | 95                    | $ 48                  |                  |                  |                  |                  |                  |
| 2019 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 188                                  | 202                   | 215                   | 224                   |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 41                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 14                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 122                                | 115                   | 94                    | $ 49                  |                  |                  |                  |                  |                  |                  |
| 2020 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 172                                  | 190                   | 204                   |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 55                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 11                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 82                                 | 68                    | $ 37                  |                  |                  |                  |                  |                  |                  |                  |
| 2021 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 190                                  | 202                   |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 49                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 11                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 82                                 | $ 37                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2022 | Workers' Compensation Line | Commercial Insurance                                           |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                      | 209                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses        | $ 84                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                              | 10                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                | $ 37                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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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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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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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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