| **Claims Development - Commercial Casualty (Details) reported_claim in Thousands, $ in Millions**       | Dec. 31, 2019 USD ($) reported_claim | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) |
|---------------------------------------------------------------------------------------------------------|--------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | $ 2,739                              |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                       | 5,799                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Commercial Casualty Line | Commercial Insurance                                                         |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 4,910                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | 2,863                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                  | 88                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                       | 2,135                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010 | Commercial Casualty Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 336                                  | $ 341                 | $ 343                 | $ 349                 | $ 348                 | $ 347                 | $ 349                 | $ 378                 | $ 394                 | $ 495                 |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 11                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 321                                | 318                   | 314                   | 300                   | 285                   | 256                   | 203                   | 159                   | 92                    | $ 33                  |
| 2011 | Commercial Casualty Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 368                                  | 365                   | 366                   | 380                   | 375                   | 377                   | 377                   | 404                   | 466                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 11                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 19                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 337                                | 325                   | 315                   | 298                   | 266                   | 227                   | 149                   | 93                    | $ 27                  |                  |
| 2012 | Commercial Casualty Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 397                                  | 399                   | 404                   | 394                   | 394                   | 417                   | 414                   | 466                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 14                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 18                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 364                                | 346                   | 330                   | 288                   | 232                   | 170                   | 88                    | $ 27                  |                  |                  |
| 2013 | Commercial Casualty Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 391                                  | 407                   | 413                   | 416                   | 431                   | 443                   | 448                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 31                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 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,                           | 465                                  | 479                   | 476                   | 479                   | 496                   | 503                   |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 34                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 21                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 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,                           | 508                                  | 516                   | 529                   | 526                   | 533                   |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 78                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 21                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 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,                           | 555                                  | 557                   | 574                   | 563                   |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 120                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 21                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 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,                           | 577                                  | 597                   | 610                   |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 160                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 234                                | 122                   | $ 48                  |                  |                  |                  |                  |                  |                  |                  |
| 2018 | Commercial Casualty Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 641                                  | 650                   |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 272                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 20                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 148                                | $ 44                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-Duration Insurance Contract, Accident Year 2019 | Commercial Casualty Line | Commercial Insurance |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                           | 672                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses             | $ 440                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                   | 15                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                     | $ 39                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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