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Inventories - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Inventory Disclosure [Abstract] |  |  |
Lower of cost or net realizable value reserves | $ 8.8 | $ 9.7 |
Reduction in inventory cost for interest received from manufacturers | 7.4 | 10.7 |
Reduction in new vehicle cost of sales for interest assistance received related to vehicles sold | $ 47.3 | $ 49.1 | $ 47.3

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