Leases and Rentals (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Leases Rentals And Outsourced Service Arrangements [Line Items]
Rental expense for operating leases	$ 165	$ 150	$ 140
Future net minimum rental payments under non-cancelable operating leases	700
Future minimum rental payments under non-cancelable operating leases payable in 2016	127
Future minimum rental payments under non-cancelable operating leases payable in 2017	121
Future minimum rental payments under non-cancelable operating leases payable in 2018	100
Future minimum rental payments under non-cancelable operating leases payable in 2019	85
Future minimum rental payments under non-cancelable operating leases payable in 2020	77
Future minimum rental payments under non-cancelable operating leases payable 2021 and thereafter	$ 190
Minimum [Member] | Office space [Member]
Leases Rentals And Outsourced Service Arrangements [Line Items]
Term length for operating leases	1 month
Maximum [Member] | Office space [Member]
Leases Rentals And Outsourced Service Arrangements [Line Items]
Term length for operating leases	18 years