Commitments and Contingencies (Capital Lease Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capital Leased Assets [Line Items]
Debt and capital lease obligations	$ 67	$ 71
Non-rental depreciation and amortization	259	255	$ 268
Less accumulated depreciation and amortization	(14)	(16)
Property and equipment, net	9	12
Rental Equipment
Capital Leased Assets [Line Items]
Depreciation and amortization	21	20	20
Property and equipment	203	190
Less accumulated depreciation and amortization	(80)	(70)
Property and equipment, net	123	120
Non-rental Equipment
Capital Leased Assets [Line Items]
Depreciation and amortization	2	3	$ 3
Non-rental vehicles
Capital Leased Assets [Line Items]
Property and equipment	2	7
Buildings
Capital Leased Assets [Line Items]
Property and equipment	$ 21	$ 21