Reserves For Losses And Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reserves For Losses And Loss Expenses [Line Items]
Workers' Compensation Discount, Amount	$ 18,929	$ 20,357	$ 21,306
Workers' Compensation Discount, Prior Year Amount		2,308
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease)	59,175	64,971	116,866
Prior year	2,401,722	2,342,378	2,216,283
Favorable Reserve Development Net Of Premium Offsets	59,000	63,000	85,000
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, net	$ 31,000	33,000
Workers' compensation discount, percent	0.00%
Workers' compensation discount after effects of ceded reinsurance, amount	$ 640,000	699,000
Weighted Average
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	0.00%
Undiscounted [Member]
Reserves For Losses And Loss Expenses [Line Items]
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years			30,000
Discounted [Member]
Reserves For Losses And Loss Expenses [Line Items]
Prior year			12,000
Insurance
Reserves For Losses And Loss Expenses [Line Items]
Favorable Reserve Development Net Of Premium Offsets	$ 53,000	52,000	69,000
Reinsurance
Reserves For Losses And Loss Expenses [Line Items]
Favorable Reserve Development Net Of Premium Offsets	6,000	$ 11,000	$ 16,000
Workers' Compensation [Member]
Reserves For Losses And Loss Expenses [Line Items]
Workers' Compensation Discount, Amount	$ 1,907
Excess workers' compensation
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	100.00%