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Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($)	3 Months Ended
	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.

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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

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Favorable Reserve Development Net Of Premium Offsets

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Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Incurred but Not Reported (IBNR) Claims, Amount

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Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Reported Claims, Amount

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Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease)

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Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance

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Unfavorable Reserve Development Net Of Premium Offsets

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Workers' Compensation Discount, Current Year Amount

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