Insurance (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of period				$ 4,282				$ 4,164	$ 4,282	$ 4,164	$ 3,899
Provision for losses and LAE occurring in the current year									1,903	1,813	1,615
Net decrease in provision for claims of prior years	(7)	22	(26)	(19)	(27)	(34)	13	(21)	(30)	(69)	(158)
Total losses and LAE incurred									1,873	1,744	1,457
Payments for losses and LAE of:
Current year									(841)	(652)	(360)
Prior years									(1,185)	(969)	(1,116)
Total payments									(2,026)	(1,621)	(1,476)
Reserves of businesses acquired											287
Foreign - currency translation and other									0	(5)	(3)
Balance at end of period	4,129				4,282				4,129	4,282	4,164
Add back reinsurance recoverable, net of allowance	2,716				2,238				2,716	2,238	2,249
Gross unpaid losses and LAE included in the Balance Sheet	$ 6,845				$ 6,520				$ 6,845	$ 6,520	$ 6,413