| **Reconciliation of Incurred Losses and ALAE (Details) - USD ($) $ in Millions**                    | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | $ 3,312       |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 7,967         |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 405           | $ 327         | $ 277         | $ 342         |
| Unallocated loss adjustment expense reserves                                                        | 369           |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 8,336         | $ 7,229       | $ 6,677       | $ 6,088       |
| Other Short-duration Insurance Product Line                                                         |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 1,512         |          |          |          |
| Commercial Insurance | Commercial Casualty Line                                                     |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 5,932         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 3,396         |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 86            |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 2,622         |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 43            |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 2,665         |          |          |          |
| Commercial Insurance | Workers' Compensation Line                                                   |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 1,865         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 1,215         |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 311           |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 961           |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 57            |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 1,018         |          |          |          |
| Commercial Insurance | Commercial Auto Line                                                         |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 2,332         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 1,581         |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 40            |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 791           |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 5             |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 796           |          |          |          |
| Commercial Insurance | Commercial Property Line                                                     |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 3,312         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 2,774         |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 15            |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 553           |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 47            |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 600           |          |          |          |
| Personal Insurance | Personal Auto                                                                  |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 1,831         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 1,557         |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 13            |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 287           |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 28            |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 315           |          |          |          |
| Personal Insurance | Home Owner Line                                                                |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 2,275         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 1,952         |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 6             |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 329           |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 8             |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | 337           |          |          |          |
| Excess and Surplus Lines Insurance                                                                  |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense**                                              |          |          |          |          |
| Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance            | 1,272         |          |          |          |
| Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance                | 587           |          |          |          |
| Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance | 2             |          |          |          |
| Total liabilities for loss and ALAE, net of reinsurance                                             | 687           |          |          |          |
| Reinsurance recoverable on unpaid losses                                                            | 37            |          |          |          |
| Total liabilities for gross loss and loss expense reserves                                          | $ 724         |          |          |          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
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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.
+ References
No definition available.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.
+ References
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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 claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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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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Amount of liability for unallocated claim adjustment expense for short-duration insurance contracts.
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