Leases and Rentals (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Leases Rentals And Outsourced Service Arrangements [Line Items]
Net rental expense for operating leases	$ 151	$ 165	$ 150
Future net minimum rental payments under non-cancelable operating leases [Abstract]
Future net minimum rental payments under non-cancelable operating leases	650
Payable in 2017	135
Payable in 2018	119
Payable in 2019	96
Payable in 2020	81
Payable in 2021	66
Payable in 2022 and thereafter	$ 164
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