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Summary of Significant Accounting Policies and Estimates - Business and Credit Risk Concentrations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Percentage of Company's new vehicle sales volume more than which no other manufacturer accounted for	1.90%
Amounts due	$ 197,181	$ 191,749
Due from Manufacturers
Concentration Risk [Line Items]
Amounts due	$ 105,140	$ 109,599
Toyota
Concentration Risk [Line Items]
Share in company sales volume	25.20%
Share in company receivable balance	15.10%
Volkswagen
Concentration Risk [Line Items]
Share in company sales volume	13.40%
Share in company receivable balance	12.50%
BMW
Concentration Risk [Line Items]
Share in company sales volume	12.00%
Share in company receivable balance	7.40%
Ford
Concentration Risk [Line Items]
Share in company sales volume	10.90%
Share in company receivable balance	10.30%
Honda
Concentration Risk [Line Items]
Share in company sales volume	9.70%
Share in company receivable balance	4.30%
Nissan
Concentration Risk [Line Items]
Share in company sales volume	6.50%
Share in company receivable balance	5.90%
General Motors
Concentration Risk [Line Items]
Share in company sales volume	6.10%
Share in company receivable balance	14.70%
Daimler
Concentration Risk [Line Items]
Share in company sales volume	4.80%
Share in company receivable balance	12.80%
FCA US
Concentration Risk [Line Items]
Share in company sales volume	3.90%
Share in company receivable balance	4.00%
Hyundai
Concentration Risk [Line Items]
Share in company sales volume	4.00%
Share in company receivable balance	2.60%
Jaguar Land Rover
Concentration Risk [Line Items]
Share in company sales volume	1.80%
Share in company receivable balance	3.20%

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Amounts due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer) for goods or services (including trade receivables) that have been delivered or sold in the normal course of business.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(5))
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