| **Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Workers' Compensation - Commercial $ in Millions** | Dec. 31, 2019 USD ($) claim | Dec. 31, 2018 USD ($) | 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 ($) | Dec. 31, 2010 USD ($) |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 5,187                     |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | 1,208                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | 735                         | $ 732                 | $ 733                 | $ 730                 | $ 710                 | $ 698                 | $ 676                 | $ 654                 | $ 632                 | $ 583                 |
| IBNR                                                                                                                                                                                                            | $ 55                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 49,333                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2011                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 688                       | 674                   | 676                   | 676                   | 651                   | 659                   | 647                   | 641                   | $ 607                 |                  |
| IBNR                                                                                                                                                                                                            | $ 40                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 45,959                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2012                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 668                       | 671                   | 673                   | 678                   | 669                   | 659                   | 627                   | $ 601                 |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 67                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 42,586                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 561                       | 582                   | 593                   | 618                   | 592                   | 572                   | $ 537                 |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 93                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 38,688                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2014                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 446                       | 450                   | 452                   | 479                   | 480                   | $ 467                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 99                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 33,480                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 394                       | 408                   | 406                   | 431                   | $ 422                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 130                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 31,861                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 382                       | 396                   | 405                   | $ 426                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 144                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 31,945                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 421                       | 432                   | $ 440                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 138                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 33,029                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2018                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 440                       | $ 450                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 185                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 34,647                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2019                                                                                                                                                                                                            |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 452                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 257                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                             | 29,795                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
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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.
+ References
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| - Definition
Cumulative number of reported claims for short-duration insurance contracts.
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