| **Claims Development - Excess and Surplus Lines (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                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Excess and Surplus Lines Insurance                                                                     |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 1,272                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | 587                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                 | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                      | 687                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 41                                   | $ 40                  | $ 41                  | $ 41                  | $ 41                  | $ 42                  | $ 45                  | $ 54                  | $ 64                  | $ 74                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 0                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 40                                 | 39                    | 39                    | 37                    | 34                    | 32                    | 27                    | 20                    | 12                    | $ 7                   |
| 2014 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 56                                   | 58                    | 59                    | 59                    | 60                    | 64                    | 75                    | 82                    | 95                    |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 1                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 55                                 | 53                    | 51                    | 48                    | 43                    | 37                    | 27                    | 17                    | $ 9                   |                  |
| 2015 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 61                                   | 65                    | 66                    | 65                    | 67                    | 73                    | 81                    | 96                    |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 1                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 58                                 | 56                    | 54                    | 51                    | 41                    | 29                    | 19                    | $ 8                   |                  |                  |
| 2016 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 88                                   | 91                    | 90                    | 82                    | 84                    | 87                    | 93                    |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 2                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 81                                 | 75                    | 62                    | 51                    | 39                    | 21                    | $ 10                  |                  |                  |                  |
| 2017 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 91                                   | 94                    | 94                    | 95                    | 95                    | 104                   |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 5                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 77                                 | 68                    | 57                    | 41                    | 23                    | $ 11                  |                  |                  |                  |                  |
| 2018 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 107                                  | 108                   | 110                   | 109                   | 116                   |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 14                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 75                                 | 62                    | 50                    | 26                    | $ 11                  |                  |                  |                  |                  |                  |
| 2019 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 139                                  | 141                   | 135                   | 137                   |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 28                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 79                                 | 55                    | 34                    | $ 13                  |                  |                  |                  |                  |                  |                  |
| 2020 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 160                                  | 172                   | 172                   |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 51                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 56                                 | 37                    | $ 16                  |                  |                  |                  |                  |                  |                  |                  |
| 2021 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 235                                  | 217                   |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 98                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 45                                 | $ 17                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2022 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 294                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 198                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 21                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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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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| - Definition
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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| - Definition
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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