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Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserves discount	$ 42	$ 37
Increase (decrease) in estimates for claims	19	(12)
Adjustment expense	2	3
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	32	2
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets		$ 5
Favorable reserve development net of premium offsets	$ 34

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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

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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

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- Definition

Favorable Reserve Development Net Of Premium Offsets

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- Definition

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

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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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