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Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions | Dec. 31, 2017 USD ($) claims | Dec. 31, 2016 USD ($) | 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 ($) | Dec. 31, 2008 USD ($)
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Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 775 |  |  |  |  |  |  |  |  |
IBNR | 305 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 44 | $ 44 | $ 45 | $ 48 | $ 52 | $ 61 | $ 68 | $ 73 | $ 114 | $ 114
IBNR | $ 0 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 7,199 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 53 | 53 | 52 | 59 | 68 | 85 | 103 | 114 | $ 114 |
IBNR | $ 1 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 6,679 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 59 | 63 | 66 | 76 | 84 | 111 | 112 | $ 112 |  |
IBNR | $ 7 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 5,962 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 66 | 70 | 75 | 87 | 116 | 121 | $ 120 |  |  |
IBNR | $ 9 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 5,795 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 45 | 52 | 70 | 98 | 122 | $ 120 |  |  |  |
IBNR | $ 9 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 5,519 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 91 | 106 | 115 | 121 | $ 120 |  |  |  |  |
IBNR | $ 10 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 4,993 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 69 | 94 | 124 | $ 123 |  |  |  |  |  |
IBNR | $ 25 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 4,938 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 104 | 131 | $ 131 |  |  |  |  |  |  |
IBNR | $ 63 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 4,670 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 124 | $ 124 |  |  |  |  |  |  |  |
IBNR | $ 84 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 4,707 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 120 |  |  |  |  |  |  |  |  |
IBNR | $ 97 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 2,901 |  |  |  |  |  |  |  |  |

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