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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inventories (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Chassis $ 8,921 $ 7,525
Raw materials 40,021 30,109
Work in process 14,995 13,521
Finished goods 29,830 17,412
Inventories $ 93,767 $ 68,567