| **Claim and Claim Adjustment Expense Reserves (Specialty - Detail of Development Recorded) (Details) - Specialty - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                        |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                           | $ (53)         | $ (99)         | $ (127) | $ (134) |
| Medical Professional Liability                                                                                                             |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                        |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                           | 15             | 8              | 38 | 30 |
| Other Professional Liability and Management Liability                                                                                      |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                        |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                           | (45)           | (19)           | (113) | (88) |
| Surety                                                                                                                                     |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                        |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                           | (20)           | (82)           | (50) | (82) |
| Warranty                                                                                                                                   |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                        |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                           | (1)            | (1)            | (7) | 5 |
| Other                                                                                                                                      |           |           |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                        |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                                                           | $ (2)          | $ (5)          | $ 5 | $ 1 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of increase (decrease) in the liability for unpaid claims and claims adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437
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| X                                                                                                                                                 |
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