Commitments and Contingencies (Future Minimum Operating Lease Payments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Operating leases, rent expense	$ 160	$ 149	$ 139
Real Estate Leases
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	117
2019	99
2020	77
2021	58
2022	35
Thereafter	39
Total	425
Non-Rental Equipment Leases
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	43
2019	37
2020	31
2021	22
2022	15
Thereafter	9
Total	$ 157