Reserves For Losses And Loss Expenses Reserves For Losses And Loss Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reserves For Losses And Loss Expenses [Line Items]
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, net	$ 34	$ 36
Reserve for losses and loss adjustment expenses relating to asbestos and environmental claims, gross	59	51
Increase (decrease) in net incurred losses and loss expenses relating to asbestos and environmental claims	1.0	2.0	(0.6)
Net paid losses and loss expenses for asbestos and environmental claims	3	3	3
Workers' compensation weighted average discount rate, percent	4.30%
Proportional business, Statutory discount rate permitted, percent	2.30%
Workers' compensation discount after effects of ceded reinsurance, amount	$ 892	$ 898	$ 877
Minimum [Member]
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	2.30%
Maximum [Member]
Reserves For Losses And Loss Expenses [Line Items]
Workers' compensation discount, percent	6.50%