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Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Other Professional Liability and Management Liability - 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 | $ 8,398 |  |  |  |  |  |  |  |  |
IBNR | 1,897 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 818 | $ 827 | $ 845 | $ 850 | $ 880 | $ 915 | $ 924 | $ 954 | $ 933 | $ 916
IBNR | $ 26 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 16,331 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 901 | 903 | 895 | 900 | 891 | 898 | 903 | 873 | $ 829 |
IBNR | $ 32 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 17,274 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 826 | 823 | 836 | 846 | 848 | 850 | 827 | $ 825 |  |
IBNR | $ 31 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 17,805 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 898 | 910 | 944 | 948 | 933 | 904 | $ 876 |  |  |
IBNR | $ 71 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 18,643 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 832 | 833 | 870 | 876 | 894 | $ 907 |  |  |  |
IBNR | $ 73 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 18,262 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 824 | 840 | 879 | 841 | $ 844 |  |  |  |  |
IBNR | $ 83 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 17,362 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 798 | 854 | 859 | $ 841 |  |  |  |  |  |
IBNR | $ 158 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 16,984 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 832 | 851 | $ 847 |  |  |  |  |  |  |
IBNR | $ 296 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 16,603 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 859 | $ 859 |  |  |  |  |  |  |  |
IBNR | $ 426 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 17,004 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 810 |  |  |  |  |  |  |  |  |
IBNR | $ 701 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 15,206 |  |  |  |  |  |  |  |  |

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