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Inventories and Related Notes Payable - Floor Plan - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Inventory Disclosure [Abstract] |  |  |
Average interest rate for new vehicle floor plan facilities | 3.10% | 2.37% | 1.85%
Amount recognized for floor plan assistance | $ 42.2 | $ 45.3 | $ 45.0
Average interest rate for used vehicle floor plan facilities | 2.98% | 2.61% | 1.78%

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- Definition Weighted average interest rate used vehicle floor plan. + References No definition available. + Details
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