| **Claims Development - Commercial Casualty (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                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Commercial Casualty Line | Commercial Insurance                                                   |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 5,932                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | 3,396                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                            | 86                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                 | 2,622                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 381                                  | $ 385                 | $ 386                 | $ 391                 | $ 407                 | $ 413                 | $ 416                 | $ 431                 | $ 443                 | $ 448                 |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 9                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 366                                | 355                   | 348                   | 337                   | 312                   | 286                   | 232                   | 159                   | 90                    | $ 35                  |
| 2014 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 462                                  | 466                   | 469                   | 465                   | 479                   | 476                   | 479                   | 496                   | 503                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 10                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 21                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 437                                | 421                   | 409                   | 390                   | 338                   | 287                   | 172                   | 97                    | $ 34                  |                  |
| 2015 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 496                                  | 504                   | 502                   | 508                   | 516                   | 529                   | 526                   | 533                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 14                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 22                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 453                                | 424                   | 404                   | 362                   | 287                   | 200                   | 108                   | $ 38                  |                  |                  |
| 2016 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 531                                  | 538                   | 554                   | 555                   | 557                   | 574                   | 563                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 32                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 22                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 466                                | 434                   | 395                   | 331                   | 228                   | 126                   | $ 46                  |                  |                  |                  |
| 2017 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 554                                  | 555                   | 571                   | 577                   | 597                   | 610                   |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 51                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 21                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 437                                | 392                   | 320                   | 234                   | 122                   | $ 48                  |                  |                  |                  |                  |
| 2018 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 612                                  | 588                   | 622                   | 641                   | 650                   |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 77                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 22                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 441                                | 345                   | 253                   | 148                   | $ 44                  |                  |                  |                  |                  |                  |
| 2019 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 669                                  | 607                   | 643                   | 672                   |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 124                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 394                                | 259                   | 134                   | $ 39                  |                  |                  |                  |                  |                  |                  |
| 2020 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 606                                  | 629                   | 674                   |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 188                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 14                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 242                                | 102                   | $ 33                  |                  |                  |                  |                  |                  |                  |                  |
| 2021 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 697                                  | 714                   |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 338                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 13                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 123                                | $ 31                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2022 | Commercial Casualty Line | Commercial Insurance                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                            |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                     | 924                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses       | $ 613                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                             | 10                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                               | $ 37                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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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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| - 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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