Reserves For Losses And Loss Expenses (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reserves For Losses And Loss Expenses [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years, Reserve Development, Net of Premiums, Favorable (Unfavorable)	$ 85,000,000	$ 98,000,000	$ 103,000,000
Prior year	2,216,283,000	2,055,284,000	2,010,101,000
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, net		36,000,000
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, gross	56,000,000	59,000,000
Increase (decrease) in net incurred losses and loss expenses relating to asbestos and environmental claims	4,000,000	5,000,000	2,000,000
Net paid losses and loss expenses for asbestos and environmental claims	3,000,000	3,000,000	2,000,000
Workers' compensation discount after effects of ceded reinsurance, amount	746,000,000	837,000,000	867,000,000
Non-Proportional Reinsurance
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation weighted average discount rate, percent	4.20%
Non-Proportional Reinsurance | Minimum
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	2.00%
Non-Proportional Reinsurance | Maximum
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	6.50%
Proportional Reinsurance
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	2.00%
Undiscounted [Member]
Reserves For Losses And Loss Expenses [Line Items]
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years	30,000,000
Discounted [Member]
Reserves For Losses And Loss Expenses [Line Items]
Prior year	$ 12,000,000