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Inventories and Related Notes Payable - Floor Plan - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Inventory Disclosure [Abstract] |  |  |
Average interest rate for new vehicle floor plan facilities | 2.37% | 1.85% | 1.61%
Amount recognized for floor plan assistance | $ 45.3 | $ 45.0 | $ 42.1
Average interest rate for used vehicle floor plan facilities | 2.61% | 1.78% | 1.72%

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