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Revenue Recognition (Narrative) (Details) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
Revenue Recognition (Narrative) (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Deferred revenue	$ 58,000,000	$ 46,000,000	$ 58,000,000		$ 58,000,000		$ 46,000,000
Provision for doubtful accounts					27,000,000	$ 24,000,000
Contract with customer, asset	0		0		0
Contract with customer, asset, impairment losses					0
Contract with customer, liability	$ 0		0		0
Revenue recognized			0	$ 0	$ 0	0
Revenues \| Largest customer \| Customer concentration risk
Property, Plant and Equipment [Line Items]
Percentage of equipment rental revenue					1.00%		1.00%	1.00%	1.00%
Concentration risk					1.00%		1.00%	1.00%	1.00%
Accounts receivable \| Customer concentration risk
Property, Plant and Equipment [Line Items]
Percentage of equipment rental revenue	1.00%	1.00%
Concentration risk	1.00%	1.00%
Rental equipment
Property, Plant and Equipment [Line Items]
Sales revenue			140,000,000	139,000,000	$ 478,000,000	378,000,000
US \| Revenues \| Geographic Concentration Risk
Property, Plant and Equipment [Line Items]
Percentage of equipment rental revenue					92.00%
Concentration risk					92.00%
Topic 840 \| Revenues \| Product concentration risk
Property, Plant and Equipment [Line Items]
Percentage of equipment rental revenue					79.00%
Concentration risk					79.00%
Topic 840 \| Rental equipment
Property, Plant and Equipment [Line Items]
Sales revenue			0	0	$ 0	0
Topic 606 \| Rental equipment
Property, Plant and Equipment [Line Items]
Sales revenue				139,000,000		378,000,000
General rentals \| Revenues \| Product concentration risk
Property, Plant and Equipment [Line Items]
Percentage of equipment rental revenue					82.00%
Concentration risk					82.00%
General rentals \| Rental equipment
Property, Plant and Equipment [Line Items]
Sales revenue			130,000,000	130,000,000	$ 446,000,000	348,000,000
Pro Forma \| Topic 605 \| ASU 2014-09 \| Rental equipment
Property, Plant and Equipment [Line Items]
Sales revenue				(14,000,000)
Transferred at point in time \| Topic 606
Property, Plant and Equipment [Line Items]
Revenue from contract with customer			$ 0	$ 0	$ 0	$ 0