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Claim, Claim Adjustment Expense and Future Policy Benefit 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, 2021 USD ($) claim | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($)
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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 | $ 8,767 |  |  |  |  |  |  |  |  |
IBNR | 2,326 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 848 | $ 850 | $ 831 | $ 833 | $ 846 | $ 840 | $ 878 | $ 887 | $ 909 | $ 923
IBNR | $ 18 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,506 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 833 | 846 | 850 | 863 | 866 | 885 | 926 | 894 | $ 884 |
IBNR | $ 30 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,950 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 841 | 845 | 854 | 835 | 831 | 885 | 898 | $ 878 |  |
IBNR | $ 37 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,577 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 836 | 813 | 807 | 832 | 877 | 892 | $ 888 |  |  |
IBNR | $ 41 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,436 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 891 | 907 | 904 | 900 | 900 | $ 901 |  |  |  |
IBNR | $ 84 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,968 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 775 | 791 | 813 | 845 | $ 847 |  |  |  |  |
IBNR | $ 152 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,159 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 906 | 869 | 864 | $ 850 |  |  |  |  |  |
IBNR | $ 202 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,926 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 856 | 845 | $ 837 |  |  |  |  |  |  |
IBNR | $ 283 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,357 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 944 | $ 930 |  |  |  |  |  |  |  |
IBNR | $ 568 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,095 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 1,037 |  |  |  |  |  |  |  |  |
IBNR | $ 911 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 15,487 |  |  |  |  |  |  |  |  |

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