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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inventories (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]    
Chassis $ 7,525 $ 8,524
Raw materials 30,109 26,322
Work in process 13,521 11,620
Finished goods 17,412 17,670
Inventories $ 68,567 $ 64,136