Acquired Property Net (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Acquired Property [Roll Forward]
Beginning balance, January 1	$ 11,373	[1]	$ 16,173	[1]	$ 9,142
Additions	16,292		18,049		25,199
Disposals	(16,629)		(21,617)		(16,237)
Write-downs, net of recoveries	(547)		(1,232)		(1,931)
Ending balance, December 31(1)	10,489	[1]	11,373	[1]	16,173	[1]
Valuation allowance	$ 669		$ 1,000		$ 1,900

[1] Includes valuation allowance of $669 million, $1.0 billion and $1.9 billion for the years ended December 31, 2012, 2011 and 2010, respectively.