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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, 2024 USD ($) claim | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
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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 | $ 9,796 |  |  |  |  |  |  |  |  |
IBNR | 2,688 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 865 | $ 858 | $ 855 | $ 836 | $ 813 | $ 807 | $ 832 | $ 877 | $ 892 | $ 888
IBNR | $ 21 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,457 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 912 | 906 | 888 | 891 | 907 | 904 | 900 | 900 | $ 901 |
IBNR | $ 37 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,989 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 752 | 746 | 758 | 775 | 791 | 813 | 845 | $ 847 |  |
IBNR | $ 51 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,215 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 987 | 941 | 923 | 906 | 869 | 864 | $ 850 |  |  |
IBNR | $ 57 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 20,071 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 970 | 939 | 876 | 856 | 845 | $ 837 |  |  |  |
IBNR | $ 96 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,548 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 945 | 945 | 951 | 944 | $ 930 |  |  |  |  |
IBNR | $ 168 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,509 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 965 | 1,009 | 1,038 | $ 1,037 |  |  |  |  |  |
IBNR | $ 311 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,377 |  |  |  |  |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 1,084 | 1,112 | $ 1,120 |  |  |  |  |  |  |
IBNR | $ 465 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 18,376 |  |  |  |  |  |  |  |  |
2023 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 1,166 | $ 1,149 |  |  |  |  |  |  |  |
IBNR | $ 564 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 19,587 |  |  |  |  |  |  |  |  |
2024 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 1,150 |  |  |  |  |  |  |  |  |
IBNR | $ 918 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 17,921 |  |  |  |  |  |  |  |  |

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