Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2023	Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserves discount	$ 35,000,000			$ 35,000,000	$ 26,000,000
Increase (decrease) in estimates for claims				12,000,000	(20,000,000)
Adjustment expense				20,000,000	37,000,000
Liability recognized for best estimate net of reinsurance	354,000,000			354,000,000
Liability recognized for best estimate net of reinsurance, current accident year losses				670,000
Unfavorable reserve development net of premium offsets			$ 24,000,000
Favorable reserve development net of premium offsets	4,000,000	$ 4,000,000
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	299,000,000			299,000,000
Unfavorable reserve development net of premium offsets				26,000,000	33,000,000
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	$ 55,000,000			55,000,000
Unfavorable reserve development net of premium offsets					$ 4,000,000
Favorable reserve development net of premium offsets				$ 6,000,000