Operating and Administrative Expenses (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Component of Operating Other Cost and Expense [Line Items]
Minimum rental commitments under noncancellable operating leases	$ 1,583
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2013	318
2014	245
2015	201
2016	164
2017	136
Equipment For Sublease [Member]
Component of Operating Other Cost and Expense [Line Items]
Rental expense under operating leases	151	159	184
Other Rental Expense [Member]
Component of Operating Other Cost and Expense [Line Items]
Rental expense under operating leases	$ 410	$ 456	$ 453