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Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net of loss reserves discount | $ 14,000,000 | $ 11,000,000
Increase (decrease) in estimates for claims | 10,000,000 | (19,000,000)
Adjustment expense | 1,000,000 | 24,000,000
Liability recognized for best estimate net of reinsurance | 386,000,000 |
Liability recognized for best estimate net of reinsurance, reported losses | 383,000,000 |
Liability recognized for best estimate net of reinsurance, incurred but not reported (IBNR) | 3,000,000 |
Insurance |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Liability recognized for best estimate net of reinsurance | 328,000,000 |
Unfavorable reserve development net of premium offsets | 8,000,000 | 12,000,000
Reinsurance & Monoline Excess |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Liability recognized for best estimate net of reinsurance | 58,000,000 |
Unfavorable reserve development net of premium offsets |  | $ 12,000,000
Favorable reserve development net of premium offsets | $ 9,000,000 |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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/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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- Definition Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Incurred but Not Reported (IBNR) Claims, Amount + References No definition available. + Details
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- Definition Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Reported Claims, Amount + References No definition available. + Details
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- Definition Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease) + References No definition available. + Details
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