Reportable Segment Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Net Sales
Total net sales	$ 612,690	$ 435,903	$ 1,104,009	$ 798,619
Income (Loss) from operations	46,041	38,668	71,696	68,932
Assets	1,369,131	932,411	1,369,131	932,411	$ 1,351,513
External Customers
Net Sales
Total net sales	612,690	435,903	1,104,009	798,619
Intersegment Sales
Net Sales
Total net sales	0	0	0	0
Commercial Trailer Products
Net Sales
Total net sales	402,507	348,140	729,929	622,929
Income (Loss) from operations	40,784	42,154	70,265	75,546
Assets	356,309	347,042	356,309	347,042
Commercial Trailer Products | External Customers
Net Sales
Total net sales	402,484	348,127	729,881	622,877
Commercial Trailer Products | Intersegment Sales
Net Sales
Total net sales	23	13	48	52
Diversified Products
Net Sales
Total net sales	94,085	90,827	189,288	180,737
Income (Loss) from operations	4,395	5,062	9,423	9,666
Assets	353,757	366,727	353,757	366,727
Diversified Products | External Customers
Net Sales
Total net sales	88,997	87,776	177,460	175,742
Diversified Products | Intersegment Sales
Net Sales
Total net sales	5,088	3,051	11,828	4,995
Final Mile Products
Net Sales
Total net sales	121,209	0	196,668	0
Income (Loss) from operations	10,258	0	10,867	0
Assets	474,121	0	474,121	0
Final Mile Products | External Customers
Net Sales
Total net sales	121,209	0	196,668	0
Final Mile Products | Intersegment Sales
Net Sales
Total net sales	0	0	0	0
Corporate and Eliminations
Net Sales
Total net sales	(5,111)	(3,064)	(11,876)	(5,047)
Income (Loss) from operations	(9,396)	(8,548)	(18,859)	(16,280)
Assets	184,944	218,642	184,944	218,642
Corporate and Eliminations | External Customers
Net Sales
Total net sales	0	0	0	0
Corporate and Eliminations | Intersegment Sales
Net Sales
Total net sales	$ (5,111)	$ (3,064)	$ (11,876)	$ (5,047)