Leased Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Leased Assets [Line Items]
Gross rental expense	$ 417	$ 415	$ 400
Sublease rental income	(53)	(61)	(66)
Rent expense, net	364	354	334
Capital leases, minimum lease payments, 2013	15
Capital leases, minimum lease payments, 2014	15
Capital leases, minimum lease payments, 2015	11
Capital leases, minimum lease payments, 2016	8
Capital leases, minimum lease payments, 2017	6
Capital leases, minimum lease payments, 2018 and beyond	27
Capital leases, minimum lease payments	82
Capital leases, imputed interest, 2013	(1)
Capital leases, imputed interest, 2014	(3)
Capital leases, imputed interest, 2015	(2)
Capital leases, imputed interest, 2016	(2)
Capital leases, imputed interest, 2017	(1)
Capital leases, imputed interest, 2018 and beyond	(15)
Capital leases, imputed interest	(24)
Capital leases. present value, 2013	14
Capital leases. present value, 2014	12
Capital leases. present value, 2015	9
Capital leases. present value, 2016	6
Capital leases. present value, 2017	5
Capital leases. present value, 2018 and beyond	12
Capital leases. present value	58
Operating leases, minimum lease payments, 2013	322
Operating leases, minimum lease payments, 2014	249
Operating leases, minimum lease payments, 2015	197
Operating leases, minimum lease payments, 2016	155
Operating leases, minimum lease payments, 2017	117
Operating leases, minimum lease payments, 2018 and beyond	382
Operating leases, minimum lease payments	1,422
Operating leases, minimum sublease rentals, 2013	(42)
Operating leases, minimum sublease rentals, 2014	(33)
Operating leases, minimum sublease rentals, 2015	(25)
Operating leases, minimum sublease rentals, 2016	(17)
Operating leases, minimum sublease rentals, 2017	(9)
Operating leases, minimum sublease rentals, 2018 and thereafter	(5)
Operating leases, minimum sublease rentals	(131)
Operating leases, 2013	280
Operating leases, 2014	216
Operating leases, 2015	172
Operating leases, 2016	138
Operating leases, 2017	108
Operating leases, 2018 and beyond	377
Operating leases	1,291
Operating leases, imputed interest	(291)
Operating leases, present value	$ 1,000