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Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net of loss reserves discount | $ 14,000,000 | $ 14,000,000
Increase (decrease) in estimates for claims | 12,000,000 | (10,000,000)
Adjustment expense | 1,000,000 | 1,000,000
Insurance |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Unfavorable reserve development net of premium offsets | 11,000,000 | 8,000,000
Reinsurance and Monoline Excess Segment |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Unfavorable reserve development net of premium offsets |  | $ 9,000,000
Favorable reserve development net of premium offsets | $ 12,000,000 |

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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