ACQUISITIONS - Narrative (Details)	1 Months Ended							3 Months Ended		12 Months Ended
	Nov. 30, 2018 USD ($) facility	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Mar. 31, 2019 USD ($) acquisition	Apr. 01, 2018 USD ($) acquisition	Dec. 31, 2018 USD ($) acquisition
Business Acquisition [Line Items]
Number of acquisitions | acquisition								0	4	9
Total sales								$ 608,218,000	$ 551,832,000
Operating Income								35,826,000	41,758,000
Contingent consideration liability								9,400,000
Contingent consideration arrangements maximum payments amount								13,700,000
Payment of contingent consideration from a business acquisition								4,416,000	0
Business acquisitions, net of cash acquired								1,222,000	95,861,000
Decrease intangible asset adjustment								3,760,000
Increase in goodwill adjustment								10,379,000
Pro forma amortization expense									2,900,000
Acquired Entities [Member]
Business Acquisition [Line Items]
Total sales									12,600,000
Operating Income									$ 1,300,000
Metal Moulding Corporation [Member]
Business Acquisition [Line Items]
Contingent consideration liability							$ 1,400,000
Net purchase price							$ 19,900,000
Business acquisitions, net of cash acquired								19,900,000
Contingent consideration performance period							1 year
Aluminum Metals Company [Member]
Business Acquisition [Line Items]
Net purchase price							$ 17,800,000
Indiana Marine Products [Member]
Business Acquisition [Line Items]
Contingent consideration liability						$ 7,900,000		7,900,000
Net purchase price						$ 18,600,000
Business acquisitions, net of cash acquired								18,600,000
Contingent consideration performance period						3 years
Collins & Company [Member]
Business Acquisition [Line Items]
Net purchase price						$ 40,000,000.0
Decrease intangible asset adjustment								3,600,000
Increase in goodwill adjustment								3,600,000
Dehco Inc [Member]
Business Acquisition [Line Items]
Net purchase price					$ 52,800,000
Decrease intangible asset adjustment								300,000
Increase in goodwill adjustment								300,000
Dowco Inc. [Member]
Business Acquisition [Line Items]
Net purchase price				$ 56,300,000
Marine Accessories Corporation [Member]
Business Acquisition [Line Items]
Net purchase price			$ 57,000,000.0
Percentage of voting interests acquired			100.00%
Engineered Metals and Composites [Member]
Business Acquisition [Line Items]
Contingent consideration liability		$ 2,500,000
Net purchase price										$ 25,200,000
Business acquisitions, net of cash acquired										$ 24,900,000
Contingent consideration payout term		3 months
Increase (decrease) in initial purchase price								300,000
Increase to accounts payable adjustment								300,000
LaSalle [Member]
Business Acquisition [Line Items]
Net purchase price	$ 51,500,000
Increase in goodwill adjustment								6,800,000
Increase (decrease) in initial purchase price								(1,500,000)
Acquiree composition number of manufacturing and distribution centers | facility	15
Increase to working capital adjustment								1,400,000
Decrease to inventory adjustment								6,700,000
Other adjustments								100,000
Accrued Liabilities [Member]
Business Acquisition [Line Items]
Contingent consideration liability								4,900,000
Other Long-term Liabilities [Member]
Business Acquisition [Line Items]
Contingent consideration liability								$ 4,500,000