Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 11,000	$ 7,000
Increase (decrease) in estimates for claims	19,000	(4,000)
Adjustment expense	24,000	1,000
Liability recognized for best estimate net of reinsurance	344,000
Liability recognized for best estimate net of reinsurance, current accident year losses	43
Liability recognized for best estimate net of reinsurance, reported losses	339,000
Liability recognized for best estimate net of reinsurance, incurred but not reported (IBNR)	5,000
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	291,000
Favorable reserve development net of premium offsets		6,000
Unfavorable reserve development net of premium offsets	17,000
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	53,000
Unfavorable reserve development net of premium offsets	$ 7,000	$ 5,000