GOODWILL AND INTANGIBLE ASSETS - Carrying Amount of Goodwill by Segment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 29, 2018	Sep. 29, 2018	Dec. 30, 2017
Goodwill
Gross balance at December 30, 2017			$ 372,612
Accumulated impairment losses			(34,892)
Balance at December 30, 2017	$ 389,594	$ 337,720	337,720
Carrying amount of goodwill
Balance at the beginning of the period		337,720
Acquisitions		87,854
Asset impairment	(14,355)	(14,355)
Divestiture of grinding media		(15,814)
Foreign currency translation		(5,811)
Balance at the end of the period	389,594	389,594
Engineered Support Structures Segment
Goodwill
Gross balance at December 30, 2017			170,076
Accumulated impairment losses			(18,670)
Balance at December 30, 2017	187,528	151,406	151,406
Carrying amount of goodwill
Balance at the beginning of the period		151,406
Acquisitions		40,919
Asset impairment		0
Divestiture of grinding media		0
Foreign currency translation		(4,797)
Balance at the end of the period	187,528	187,528
Utility Support Structures Segment
Goodwill
Gross balance at December 30, 2017			90,248
Accumulated impairment losses			0
Balance at December 30, 2017	116,836	90,248	90,248
Carrying amount of goodwill
Balance at the beginning of the period		90,248
Acquisitions		41,432
Asset impairment		(14,355)
Divestiture of grinding media		0
Foreign currency translation		(489)
Balance at the end of the period	116,836	116,836
Coatings Segment
Goodwill
Gross balance at December 30, 2017			76,696
Accumulated impairment losses			(16,222)
Balance at December 30, 2017	60,094	60,474	60,474
Carrying amount of goodwill
Balance at the beginning of the period		60,474
Acquisitions		0
Asset impairment		0
Divestiture of grinding media		0
Foreign currency translation		(380)
Balance at the end of the period	60,094	60,094
Irrigation Segment
Goodwill
Gross balance at December 30, 2017			19,778
Accumulated impairment losses			0
Balance at December 30, 2017	25,136	19,778	19,778
Carrying amount of goodwill
Balance at the beginning of the period		19,778
Acquisitions		5,503
Asset impairment		0
Divestiture of grinding media		0
Foreign currency translation		(145)
Balance at the end of the period	25,136	25,136
Other Segments
Goodwill
Gross balance at December 30, 2017			15,814
Accumulated impairment losses			0
Balance at December 30, 2017	0	15,814	$ 15,814
Carrying amount of goodwill
Balance at the beginning of the period		15,814
Acquisitions		0
Asset impairment		0
Divestiture of grinding media		(15,814)
Foreign currency translation		0
Balance at the end of the period	$ 0	$ 0