| **Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ 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                                                                                                                     | $ 778                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | 305                          |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2009                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | 52                           | $ 53                  | $ 53                  | $ 52                  | $ 59                  | $ 68                  | $ 85                  | $ 103                 | $ 114                 | $ 114                 |
| IBNR                                                                                                                                                                            | $ 1                          |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 6,688                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2010                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 61                         | 59                    | 63                    | 66                    | 76                    | 84                    | 111                   | 112                   | $ 112                 |                  |
| IBNR                                                                                                                                                                            | $ 1                          |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 5,971                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2011                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 62                         | 66                    | 70                    | 75                    | 87                    | 116                   | 121                   | $ 120                 |                  |                  |
| IBNR                                                                                                                                                                            | $ 1                          |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 5,808                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2012                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 39                         | 45                    | 52                    | 70                    | 98                    | 122                   | $ 120                 |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 2                          |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 5,559                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 87                         | 91                    | 106                   | 115                   | 121                   | $ 120                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 6                          |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 5,039                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2014                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 60                         | 69                    | 94                    | 124                   | $ 123                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 19                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 5,036                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 79                         | 104                   | 131                   | $ 131                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 36                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 4,887                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 109                        | 124                   | $ 124                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 60                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 5,185                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 115                        | $ 120                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 78                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 4,936                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2018                                                                                                                                                                            |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                           |                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                     | $ 114                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                            | $ 101                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claims                                                                                                                                            | 3,105                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ References
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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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