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