Inventories (Details) - USD ($) $ in Thousands  | 
Sep. 30, 2018  | 
Dec. 31, 2017  | 
|---|---|---|
| Inventories | ||
| Finished goods | $ 43,480 | $ 35,547 | 
| Work-in-process | 8,704 | 7,774 | 
| Raw material and supplies | 37,260 | 28,203 | 
| Inventories | 89,444 | 71,524 | 
| Inventories - truck chassis floor plan | $ 9,495 | $ 7,711 | 
| X | ||||||||||
- Definition Amount of inventory, truck chassis floor plan expected to be sold, or consumed within one year or operating cycle, if longer. No definition available. 
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| X | ||||||||||
- References No definition available. 
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| X | ||||||||||
- Definition Amount before valuation and LIFO reserves of completed merchandise or goods expected to be sold within one year or operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
- Definition Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
- Definition Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
- Definition Amount before valuation and LIFO reserves of merchandise or goods in the production process expected to be completed within one year or operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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