Leased Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Leases [Abstract]
Gross rental expense	$ 324	$ 387	$ 400
Sublease rental income	(33)	(40)	(43)
Rent expense, net	291	$ 347	$ 357
Capital leases, minimum lease payments, due in 2016	12
Capital leases, minimum lease payments, due in 2017	11
Capital leases, minimum lease payments, due in 2018	8
Capital leases, minimum lease payments, due in 2019	5
Capital leases, minimum lease payments, due in 2020	3
Capital leases, minimum lease payments, due thereafter	41
Capital leases, minimum lease payments	80
Capital leases, imputed interest, due in 2016	(4)
Capital leases, imputed interest, due in 2017	(3)
Capital leases, imputed interest, due in 2018	(3)
Capital leases, imputed interest, due in 2019	(3)
Capital leases, imputed interest, due in 2020	(2)
Capital leases, imputed interest, due thereafter	(17)
Capital leases, imputed interest	(32)
Capital leases. present value, due in 2016	8
Capital leases. present value, due in 2017	8
Capital leases. present value, due in 2018	5
Capital leases. present value, due in 2019	2
Capital leases. present value, due in 2020	1
Capital leases. present value, due thereafter	24
Capital leases. present value	48
Operating leases, minimum lease payments, due in 2016	279
Operating leases, minimum lease payments, due in 2017	215
Operating leases, minimum lease payments, due in 2018	158
Operating leases, minimum lease payments, due in 2019	118
Operating leases, minimum lease payments, due in 2020	94
Operating leases, minimum lease payments, due thereafter	299
Operating leases, minimum lease payments	1,163
Operating leases, minimum sublease rentals, due in 2016	(26)
Operating leases, minimum sublease rentals, due in 2017	(17)
Operating leases, minimum sublease rentals, due in 2018	(10)
Operating leases, minimum sublease rentals, due in 2019	(6)
Operating leases, minimum sublease rentals, due in 2020	(3)
Operating leases, minimum sublease rentals, due thereafter	(7)
Operating leases, minimum sublease rentals	(69)
Operating leases, due in 2016	253
Operating leases, due in 2017	198
Operating leases, due in 2018	148
Operating leases, due in 2019	112
Operating leases, due in 2020	91
Operating leases, due thereafter	292
Operating leases	1,094
Operating leases, imputed interest	(208)
Operating leases, present value	$ 886