| **Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ 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                                                                                                                                                                                | $ 4,685                     |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | 853                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | 369                         | $ 356                 | $ 360                 | $ 365                 | $ 395                 | $ 415                 | $ 426                 | $ 423                 | $ 412                 | $ 402                 |
| IBNR                                                                                                                                                                                                                                       | $ 1                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 14,624                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2011                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 439                       | 437                   | 437                   | 434                   | 439                   | 468                   | 443                   | 437                   | $ 429                 |                  |
| IBNR                                                                                                                                                                                                                                       | $ 2                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 16,526                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2012                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 499                       | 493                   | 484                   | 493                   | 498                   | 508                   | 469                   | $ 464                 |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 8                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 17,724                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 545                       | 535                   | 525                   | 513                   | 500                   | 479                   | $ 462                 |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 27                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 19,510                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2014                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 529                       | 535                   | 530                   | 537                   | 489                   | $ 450                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 16                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 19,723                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 488                       | 494                   | 510                   | 499                   | $ 433                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 29                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 18,029                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 499                       | 485                   | 487                   | $ 427                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 63                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 15,823                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 458                       | 449                   | $ 412                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 127                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 14,636                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2018                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 429                       | $ 404                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 216                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 13,760                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2019                                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                                | $ 430                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                                       | $ 364                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                                        | 10,467                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ References
No definition available.
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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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