Losses And Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2007
Extraordinary Item [Line Items]
Increase in provision for insured events	$ 18
Decrease in provision for insured events		13	58
Unfavorable development		3
Favorable development				5
Catastrophe [Member]
Extraordinary Item [Line Items]
Pre-tax catastrophe losses	$ 18	$ 25	$ 0