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Summary of Significant Accounting Policies and Estimates - Inventories (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Inventory [Line Items]          
Reduction in inventory cost for interest received from manufacturers $ 9.7   $ 9.7 $ 9.1  
Reduction in new vehicle cost of sales for interest assistance received related to vehicles sold     $ 47.3 $ 48.9 $ 49.2
Number of days' supply of used vehicles     39 days 39 days  
Minimum          
Inventory [Line Items]          
Interest assistance received 77.10%        
Maximum          
Inventory [Line Items]          
Interest assistance received   116.60%