| **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, 2018 USD ($) claims | Dec. 31, 2017 USD ($) | 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 ($) | Dec. 31, 2009 USD ($) |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 5,445                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | 1,130                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2009                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | 650                          | $ 649                 | $ 638                 | $ 631                 | $ 626                 | $ 616                 | $ 611                 | $ 609                 | $ 599                 | $ 592                 |
| IBNR                                                                                                                                                                                                            | $ 46                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 51,822                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 732                        | 733                   | 730                   | 710                   | 698                   | 676                   | 654                   | 632                   | $ 583                 |                  |
| IBNR                                                                                                                                                                                                            | $ 44                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 49,106                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2011                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 674                        | 676                   | 676                   | 651                   | 659                   | 647                   | 641                   | $ 607                 |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 27                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 45,637                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2012                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 671                        | 673                   | 678                   | 669                   | 659                   | 627                   | $ 601                 |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 59                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 42,477                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 582                        | 593                   | 618                   | 592                   | 572                   | $ 537                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 86                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 38,665                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2014                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 450                        | 452                   | 479                   | 480                   | $ 467                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 104                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 33,465                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 408                        | 406                   | 431                   | $ 422                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 146                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 31,828                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 396                        | 405                   | $ 426                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 171                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 31,905                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 432                        | $ 440                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 171                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 32,811                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2018                                                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                     | $ 450                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                            | $ 276                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                                                            | 30,399                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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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.
+ References
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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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