SEGMENTS - Reportable Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Net sales
Total net sales	$ 626,053	$ 612,690	$ 1,159,227	$ 1,104,009
Income (loss) from operations	47,538	46,041	72,736	71,696
Assets	1,374,829	1,369,131	1,374,829	1,369,131	$ 1,304,393
Commercial Trailer Products
Net sales
Total net sales	400,401	402,484	740,546	729,881
Diversified Products
Net sales
Total net sales	90,835	88,997	183,015	177,460
Final Mile Products
Net sales
Total net sales	134,817	121,209	235,666	196,668
Corporate and Eliminations
Net sales
Total net sales	(6,654)	(5,111)	(15,022)	(11,876)
Income (loss) from operations	(10,512)	(9,396)	(21,548)	(18,859)
Assets	182,019	184,944	182,019	184,944
Corporate and Eliminations | Commercial Trailer Products
Net sales
Total net sales	463	23	1,363	48
Corporate and Eliminations | Diversified Products
Net sales
Total net sales	6,191	5,088	13,659	11,828
Corporate and Eliminations | Final Mile Products
Net sales
Total net sales	0	0	0	0
Operating Segments | Commercial Trailer Products
Net sales
Total net sales	400,864	402,507	741,909	729,929
Income (loss) from operations	39,918	40,784	66,239	70,265
Assets	358,880	356,309	358,880	356,309
Operating Segments | Diversified Products
Net sales
Total net sales	97,026	94,085	196,674	189,288
Income (loss) from operations	8,911	4,395	16,955	9,423
Assets	337,197	353,757	337,197	353,757
Operating Segments | Final Mile Products
Net sales
Total net sales	134,817	121,209	235,666	196,668
Income (loss) from operations	9,221	10,258	11,090	10,867
Assets	$ 496,733	$ 474,121	$ 496,733	$ 474,121