Debt (Fair Value of Debt) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, Fair Value Disclosure	$ 5,621		$ 5,863
Subsidiaries Notes Disclosure	0		1,075
Notes Payable, Other Payables, Disclosure	1,670	[1]	1,599	[1]
Subsidiaries Term Loan	1,180		0
Loans Payable, Fair Value Disclosure	2,452		2,489
Debt Excluding Capital Leases	10,923		11,026
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, Fair Value Disclosure	5,304		5,303
Subsidiaries Notes Disclosure	0		978
Notes Payable, Other Payables, Disclosure	1,723	[1]	1,629	[1]
Subsidiaries Term Loan	1,197		0
Loans Payable, Fair Value Disclosure	2,450		2,463
Debt Excluding Capital Leases	$ 10,674		$ 10,373

[1] Excludes a lease that is accounted for as a failed sale-leaseback transaction under U.S. GAAP.