Unpaid Losses and Loss Expenses (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of the beginning and ending liability for unpaid losses and loss expenses [Abstract]
Gross liability, beginning of year	$ 22,718	$ 22,839	$ 22,367
Reinsurance recoverable, beginning of year	1,817	2,053	2,212
Net liability, beginning of year	20,901	20,786	20,155
Net incurred losses and loss expenses related to [Abstract]
Current year	8,174	7,245	7,030
Prior years	(767)	(746)	(762)
Net incurred losses and loss expenses, total	7,407	6,499	6,268
Net payments for losses and loss expenses related to [Abstract]
Current year	2,746	2,280	1,943
Prior years	4,300	4,074	4,063
Net payments for losses and loss expenses, total	7,046	6,354	6,006
Foreign currency translation effect	67	(30)	369
Net liability, end of year	21,329	20,901	20,786
Reinsurance recoverable, end of year	1,739	1,817	2,053
Gross liability, end of year	$ 23,068	$ 22,718	$ 22,839