SEGMENTS - Reportable Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Net sales
Total net sales	$ 449,422	$ 339,153	$ 841,425	$ 726,227
Income (loss) from operations	22,684	5,999	33,899	(104,057)
Assets	1,180,500	1,142,265	1,180,500	1,142,265	$ 1,161,470
Commercial Trailer Products
Net sales
Total net sales	296,103	232,247	542,874	483,192
Diversified Products
Net sales
Total net sales	72,296	56,074	140,463	131,933
Final Mile Products
Net sales
Total net sales	81,023	50,832	158,088	111,102
Corporate and Eliminations
Net sales
Total net sales	(4,521)	(7,884)	(11,300)	(15,013)
Income (loss) from operations	(12,192)	(8,273)	(23,984)	(20,089)
Assets	238,080	134,351	238,080	134,351
Corporate and Eliminations | Commercial Trailer Products
Net sales
Total net sales	239	7	1,177	37
Corporate and Eliminations | Diversified Products
Net sales
Total net sales	4,282	7,877	10,123	14,976
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	296,342	232,254	544,051	483,229
Income (loss) from operations	32,299	18,599	53,164	34,470
Assets	310,179	347,072	310,179	347,072
Operating Segments | Diversified Products
Net sales
Total net sales	76,578	63,951	150,586	146,909
Income (loss) from operations	5,824	2,242	11,913	(3,828)
Assets	256,905	289,927	256,905	289,927
Operating Segments | Final Mile Products
Net sales
Total net sales	81,023	50,832	158,088	111,102
Income (loss) from operations	(3,247)	(6,569)	(7,194)	(114,610)
Assets	$ 375,336	$ 370,915	$ 375,336	$ 370,915