Unpaid Losses and Loss Expenses (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Md	Dec. 31, 2010	Dec. 31, 2009
Unpaid Losses And Loss Expenses (Textual) [Abstract]
Favorable development on net unpaid losses and loss expenses established as of the previous year end	$ 767	$ 746	$ 762
Favorable development experienced in the professional liability classes other than fidelity	310	315	340
Favorable development experienced in the professional liability classes other than fidelity, outside the United States		190	110
Aggregate favorable development experienced in the personal and commercial liability classes	355	265	150
Aggregate favorable development experienced in the homeowners and commercial property classes			160
Adverse development for losses related to toxic waste claims		61	90
Aggregate favorable development experienced in the personal and commercial property classes	80	110
Adverse development experienced in the fidelity class	70	70
Favorable development experienced in the personal automobile business	30	40	30
Favorable development experienced in the surety business	15	40	35
Favorable development experienced in the runoff of the reinsurance assumed business	30	25	55
Number of manufacturers and distributors of asbestos products that filed for bankruptcy protection	90
Adverse development for losses related to asbestos and toxic waste claims	$ 72