Acquired Property Net (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Acquired Property, Net [Roll Forward]
Acquired Property, Net Beginning Balance	$ 10,489		$ 11,373
Acquired Property, Net Additions	3,676		3,705
Acquired Property, Net Disposals	(3,901)		(4,300)
Valuation Allowance, Write-downs, net of recoveries	(115)		(159)
Acquired Property, Net Ending Balance	10,149	[1]	10,619	[1]
Valuation allowance, ending balance	$ 570		$ 790

[1] Includes valuation allowance of $570 million and $790 million as of March 31, 2013 and 2012, respectively.