GOODWILL AND INTANGIBLE ASSETS - Carrying Amount of Goodwill by Segment (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 30, 2017
Goodwill
Goodwill, Gross		$ 372,612,000
Accumulated impairment losses		(34,892,000)
Carrying amount of goodwill
Balance at the beginning of the period	$ 337,720,000
Acquisitions	(3,922,000)
Assets held for sale	(16,420,000)
Foreign currency translation	2,652,000
Balance at the end of the period	327,874,000
Engineered Support Structures Segment
Goodwill
Goodwill, Gross		170,076,000
Accumulated impairment losses		(18,670,000)
Carrying amount of goodwill
Balance at the beginning of the period	151,406,000
Acquisitions	0
Assets held for sale	0
Foreign currency translation	1,984,000
Balance at the end of the period	153,390,000
Utility Support Structures Segment
Goodwill
Goodwill, Gross		90,248,000
Accumulated impairment losses		0
Carrying amount of goodwill
Balance at the beginning of the period	90,248,000
Acquisitions	0
Assets held for sale	0
Foreign currency translation	402,000
Balance at the end of the period	90,650,000
Coatings Segment
Goodwill
Goodwill, Gross		76,696,000
Accumulated impairment losses		(16,222,000)
Carrying amount of goodwill
Balance at the beginning of the period	60,474,000
Acquisitions	0
Assets held for sale	0
Foreign currency translation	(200,000)
Balance at the end of the period	60,274,000
Irrigation Segment
Goodwill
Goodwill, Gross		19,778,000
Accumulated impairment losses		0
Carrying amount of goodwill
Balance at the beginning of the period	19,778,000
Acquisitions	(3,922,000)
Assets held for sale	0
Foreign currency translation	(140,000)
Balance at the end of the period	23,560,000
Other Segments
Goodwill
Goodwill, Gross		15,814,000
Accumulated impairment losses		$ 0
Carrying amount of goodwill
Balance at the beginning of the period	15,814,000
Acquisitions	0
Assets held for sale	(16,420,000)
Foreign currency translation	606,000
Balance at the end of the period	0
Valmont SM
Carrying amount of goodwill
Balance at the end of the period	$ 15,245