Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserves discount	$ 37,000,000	$ 35,000,000
Increase (decrease) in estimates for claims	12,000,000	(12,000,000)
Adjustment expense	(3,000,000)	20,000,000
Liability recognized for best estimate net of reinsurance	382,000,000
Liability recognized for best estimate net of reinsurance, reported losses	379,000,000
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	325,000,000
Unfavorable reserve development net of premium offsets	2,000,000	21,000,000
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	57,000,000
Favorable reserve development net of premium offsets	$ 5,000,000	$ 1,000,000