| **Claims Development - Workers Compensation (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                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Workers' Compensation Line | Commercial Insurance                                                         |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 2,061                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | 1,433                                |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                    | 295                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                         | 923                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010 | Workers' Compensation Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 237                                  | $ 236                 | $ 237                 | $ 240                 | $ 239                 | $ 240                 | $ 242                 | $ 248                 | $ 274                 | $ 283                 |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 19                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 26                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 209                                | 207                   | 204                   | 202                   | 198                   | 192                   | 181                   | 164                   | 134                   | $ 67                  |
| 2011 | Workers' Compensation Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 228                                  | 229                   | 231                   | 236                   | 239                   | 242                   | 246                   | 251                   | 284                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 17                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 24                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 197                                | 195                   | 192                   | 190                   | 186                   | 177                   | 161                   | 131                   | $ 65                  |                  |
| 2012 | Workers' Compensation Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 208                                  | 209                   | 211                   | 213                   | 220                   | 234                   | 245                   | 265                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 21                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 21                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 180                                | 178                   | 175                   | 171                   | 162                   | 147                   | 121                   | $ 62                  |                  |                  |
| 2013 | Workers' Compensation Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 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                                                       | $ 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,                             | 198                                  | 201                   | 203                   | 214                   | 233                   | 261                   |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 18                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 19                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 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,                             | 179                                  | 195                   | 208                   | 220                   | 246                   |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 33                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 17                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 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,                             | 188                                  | 206                   | 218                   | 230                   |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 31                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 16                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 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,                             | 190                                  | 208                   | 218                   |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 56                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 15                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 106                                | 88                    | $ 45                  |                  |                  |                  |                  |                  |                  |                  |
| 2018 | Workers' Compensation Line | Commercial Insurance                                                  |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 207                                  | 222                   |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 65                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 15                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 95                                 | $ 48                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Short-Duration Insurance Contract, Accident Year 2019 | Workers' Compensation Line | Commercial Insurance |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                    |                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                             | 224                                  |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses               | $ 90                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                     | 13                                   |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                       | $ 49                                 |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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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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