| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Specialty - Other Professional and Management Liability (Details) - USD ($) $ in Millions** | 12 Months Ended |         |         |          |        |        |        |        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|---------|---------|----------|--------|--------|--------|--------|
||
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years                                                                                                                                          | $ 4,167         | $ 4,089 | $ 4,355 |     |   |   |   |   |   |   |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                                                                                                               | 18,249          | 18,576  | 18,927  | $ 19,117 |   |   |   |   |   |   |
| Specialty - Other Professional and Management Liability [Member]                                                                                                                                                             |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 5,475           |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                  | 2,887           |    |    |     |   |   |   |   |   |   |
| Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years                                                                                                                                          | 104             |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense                                                                              | 72              |    |    |     |   |   |   |   |   |   |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                                                                                                               | 3,063           |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2007 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 614             | 593     | 585     | 564      | $ 524  | $ 472  | $ 397  | $ 307  | $ 162 | $ 32 |
| Short-duration Insurance Contracts, Accident Year 2008 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 741             | 719     | 678     | 641      | 600    | 515    | 376    | 181    | $ 39 |   |
| Short-duration Insurance Contracts, Accident Year 2009 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 798             | 769     | 719     | 638      | 550    | 358    | 195    | $ 37   |   |   |
| Short-duration Insurance Contracts, Accident Year 2010 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 721             | 670     | 630     | 541      | 404    | 203    | $ 31   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2011 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 726             | 682     | 604     | 502      | 313    | $ 71   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2012 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 648             | 570     | 398     | 248      | $ 57   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2013 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 583             | 426     | 240     | $ 51     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2014 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 375             | 212     | $ 51    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2015 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | 209             | $ 48    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2016 [Member] | Specialty - Other Professional and Management Liability [Member]                                                                                           |            |    |    |     |   |   |   |   |   |   |
| **Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]**                                                                                                                                      |            |    |    |     |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net                                                                                                                       | $ 60            |    |    |     |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Table]
+ References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of liability for unallocated claim adjustment expense for short-duration insurance contracts.
+ References
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