Reserves For Losses And Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reserves For Losses And Loss Expenses [Line Items]
Liability recognized for best estimate net of reinsurance	$ 171,000
Liability recognized for best estimate net of reinsurance, reported losses	95,000
Liability recognized for best estimate net of reinsurance, incurred but not reported (IBNR)	76,000
Favorable reserve development net of premium offsets	16,180	$ 19,432	$ 38,807
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, net	19,000	24,000
Workers' compensation discount, amount	10,000	20,000	24,000
Workers' Compensation Discount, Prior Year Amount		1,731,000
Workers' compensation discount after effects of ceded reinsurance, amount	$ 483,000	530,000
Minimum
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	0.70%
Maximum
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	6.50%
Weighted Average
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	3.60%
Discounted [Member]
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	3.00%
Insurance
Reserves For Losses And Loss Expenses [Line Items]
Liability recognized for best estimate net of reinsurance	$ 161,000
Favorable reserve development net of premium offsets	24,000	21,000	19,000
Reinsurance & Monoline Excess
Reserves For Losses And Loss Expenses [Line Items]
Liability recognized for best estimate net of reinsurance	10,000
Favorable reserve development net of premium offsets			$ 20,000
Unfavorable reserve development net of premium offsets	8,000	$ 2,000
Short-tail lines
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, amount	$ 1,655,000
Excess workers' compensation
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	97.00%