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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% |  |  |

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- Definition Manufacturer floorplan assistance recognized as reduction of cost of sales. + References No definition available. + Details
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- Definition Manufacturer floorplan assistance recognized as reduction of inventory cost. + References No definition available. + Details
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