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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, 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 | $ 906 |  |  |  |  |  |  |  |  |
IBNR | 427 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 45 | $ 45 | $ 45 | $ 53 | $ 58 | $ 63 | $ 79 | $ 104 | $ 131 | $ 131
IBNR | $ 2 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 5,103 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 43 | 43 | 58 | 64 | 67 | 84 | 109 | 124 | $ 124 |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 5,577 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 67 | 67 | 66 | 71 | 84 | 103 | 115 | $ 120 |  |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 5,909 |  |  |  |  |  |  |  |  |
2018 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 49 | 51 | 56 | 62 | 91 | 108 | $ 114 |  |  |
IBNR | $ 6 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 6,297 |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 82 | 82 | 87 | 98 | 112 | $ 119 |  |  |  |
IBNR | $ 8 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 6,229 |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 57 | 67 | 81 | 119 | $ 128 |  |  |  |  |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 4,827 |  |  |  |  |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 91 | 110 | 129 | $ 137 |  |  |  |  |  |
IBNR | $ 45 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 4,884 |  |  |  |  |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 132 | 158 | $ 155 |  |  |  |  |  |  |
IBNR | $ 74 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 4,893 |  |  |  |  |  |  |  |  |
2023 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 169 | $ 175 |  |  |  |  |  |  |  |
IBNR | $ 138 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 4,378 |  |  |  |  |  |  |  |  |
2024 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 171 |  |  |  |  |  |  |  |  |
IBNR | $ 145 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claim | 2,940 |  |  |  |  |  |  |  |  |

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