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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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