Reserves For Losses And Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reserves For Losses And Loss Expenses [Line Items]
Favorable reserve development net of premium offsets	$ 19,432	$ 38,807	$ 37,327
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, net	24,000	28,000
Workers' compensation discount, amount	20,000	24,000	22,000
Workers' Compensation Discount, Prior Year Amount		1,793,000
Prior year	2,673,803	2,700,077	2,562,550
Workers' compensation discount after effects of ceded reinsurance, amount	$ 530,000	563,000
Minimum
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	2.00%
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	3.70%
Discounted [Member] | Weighted Average
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	3.00%
Insurance
Reserves For Losses And Loss Expenses [Line Items]
Favorable reserve development net of premium offsets	$ 21,000	19,000	30,000
Reinsurance & Monoline Excess
Reserves For Losses And Loss Expenses [Line Items]
Favorable reserve development net of premium offsets		$ 20,000	$ 7,000
Unfavorable Reserve Development Net Of Premium Offsets	2,000
Reinsurance & Monoline Excess | Minimum
Reserves For Losses And Loss Expenses [Line Items]
Reinsurance Reserves, Ogden Discount Rate			2.50%
Reinsurance & Monoline Excess | Maximum
Reserves For Losses And Loss Expenses [Line Items]
Reinsurance Reserves, Ogden Discount Rate			0.75%
Workers' compensation
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, amount	$ 1,731,000
Excess workers' compensation
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	97.00%