Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt - 2016 Credit Facilities (Details)

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Long-Term Debt - 2016 Credit Facilities (Details) - USD ($)
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
2016 Revolving Credit Facility    
Line of Credit Facility [Line Items]    
2016 Revolving Credit Facility $ 13,526,000 $ 75,000,000
Revolving Credit Facility | 2016 Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Current borrowing capacity 250,000,000.0  
Maximum borrowing capacity 300,000,000.0  
Borrowing base 222,500,000  
Letters of credit outstanding amount 16,200,000  
Borrowing availability amount 192,800,000  
Revolving Credit Facility | 2016 Credit Facilities    
Line of Credit Facility [Line Items]    
2016 Revolving Credit Facility 13,500,000  
Revolving Credit Facility | 2016 New Vehicle Floor Plan Facility and 2016 Used Vehicle Floor Plan Facility    
Line of Credit Facility [Line Items]    
Current borrowing capacity 1,015,000,000.000  
Maximum borrowing capacity $ 1,265,000,000  
Maximum aggregate commitments allocated to commitments under the 2016 Used Vehicle Floor Plan Facility 30.00%