Reserves For Losses And Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reserves For Losses And Loss Expenses [Line Items]
Workers' Compensation Discount, Amount	$ 24,381	$ 22,064	$ 18,929
Workers' Compensation Discount, Prior Year Amount		1,855,000
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease)	3,738	32,132	59,175
Prior year	2,700,077	2,562,550	2,401,722
Favorable Reserve Development Net Of Premium Offsets	39,000	37,000	59,000
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, net	28,000	30,000
Workers' compensation discount after effects of ceded reinsurance, amount	$ 563,000	591,000
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	3.80%
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	$ 43,000	68,000	53,000
Reinsurance
Reserves For Losses And Loss Expenses [Line Items]
Favorable Reserve Development Net Of Premium Offsets			$ 6,000
Unfavorable Reserve Development Net Of Premium Offsets	4,000	$ 31,000
Reinsurance Reserves, Ogden Discount Rate		(0.75%)	2.50%
Workers' compensation
Reserves For Losses And Loss Expenses [Line Items]
Workers' Compensation Discount, Amount	$ 1,793,000
Excess workers' compensation
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	97.00%