Vehicle Service Agreement Liability Reconciliation of the changes in the vehicle service agreement (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Product Warranty Liability [Line Items]
Extended Product Warranty Accrual, Increase for Warranties Issued	$ 15,904	$ 14,129
Deferred Revenue, Revenue Recognized	(12,700)
Extended Product Warranty Accrual, Decrease for Payments	(4,138)	(4,129)
Standard Product Warranty Accrual, Increase (Decrease) for Preexisting Warranties	(252)	(139)
Extended Product Warranty Accrual	42,921	40,005	$ 40,292	$ 38,713
Vehicle service agreement fees [Member]
Product Warranty Liability [Line Items]
Deferred Revenue, Revenue Recognized	(13,091)	(12,635)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	4,206	$ 4,066
Extended Product Warranty Accrual	$ 42,921		$ 40,292

Amount of revenue recognized that was previously reported as deferred or unearned revenue.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response))
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

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Amount as of the balance sheet date of the aggregate extended product warranty liability. Does not include the ending balance for the standard product warranty liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)(1),(c)(5)
-URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249

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Amount of decrease in the extended product warranty accrual from payments made in cash or in kind to satisfy claims under the terms of the extended product warranty. Excludes standard product warranties.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249

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Amount of increase in the extended product warranty accrual from warranties issued. Excludes standard product warranties.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249

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Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

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Amount of increase (decrease) in the standard product warranty accrual from changes in estimates attributable to preexisting product warranties. Excludes extended product warranties.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 8
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-URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249

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