ACQUISITIONS - Fair Value of Assets Acquired, Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Mar. 31, 2018	Feb. 28, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Goodwill	$ 281,734	$ 208,044	$ 109,893
Contingent consideration liability	13,800
Business acquisitions	343,347	251,851	138,915
Metal Moulding Corporation [Member]
Business Acquisition [Line Items]
Trade receivables	1,463
Inventories	2,324
Property, plant and equipment	2,085
Prepaid expenses & other	0
Intangible assets	8,540
Goodwill	7,668
Less: Total liabilities	827
Less: Deferred tax liability, net	0
Total net assets acquired	21,253
Contingent consideration liability	1,400					$ 1,400
Business acquisitions	19,900
Aluminum Metals Company [Member]
Business Acquisition [Line Items]
Trade receivables	3,942
Inventories	5,623
Property, plant and equipment	2,321
Prepaid expenses & other	39
Intangible assets	6,550
Goodwill	1,755
Less: Total liabilities	2,463
Less: Deferred tax liability, net	0
Total net assets acquired	17,767
Indiana Marine Products [Member]
Business Acquisition [Line Items]
Trade receivables	1,962
Inventories	4,267
Property, plant and equipment	1,306
Prepaid expenses & other	13
Intangible assets	12,860
Goodwill	8,979
Less: Total liabilities	2,888
Less: Deferred tax liability, net	0
Total net assets acquired	26,499
Contingent consideration liability	7,900				$ 7,900
Business acquisitions	18,600
Collins & Company [Member]
Business Acquisition [Line Items]
Trade receivables	2,830
Inventories	9,903
Property, plant and equipment	1,125
Prepaid expenses & other	5
Intangible assets	22,000
Goodwill	6,730
Less: Total liabilities	2,586
Less: Deferred tax liability, net	0
Total net assets acquired	40,007
Dehco Inc [Member]
Business Acquisition [Line Items]
Trade receivables	4,771
Inventories	16,923
Property, plant and equipment	13,755
Prepaid expenses & other	208
Intangible assets	14,200
Goodwill	6,330
Less: Total liabilities	3,392
Less: Deferred tax liability, net	0
Total net assets acquired	52,795
Dowco Inc. [Member]
Business Acquisition [Line Items]
Trade receivables	4,053
Inventories	4,498
Property, plant and equipment	5,910
Prepaid expenses & other	1,240
Intangible assets	34,379
Goodwill	10,423
Less: Total liabilities	4,198
Less: Deferred tax liability, net	0
Total net assets acquired	56,305
Marine Accessories Corporation [Member]
Business Acquisition [Line Items]
Trade receivables	3,054
Inventories	6,815
Property, plant and equipment	8,000
Prepaid expenses & other	284
Intangible assets	32,733
Goodwill	19,264
Less: Total liabilities	4,290
Less: Deferred tax liability, net	8,839
Total net assets acquired	57,021
Engineered Metals And Composites [Member]
Business Acquisition [Line Items]
Trade receivables	623
Inventories	1,577
Property, plant and equipment	2,500
Prepaid expenses & other	0
Intangible assets	15,750
Goodwill	8,073
Less: Total liabilities	798
Less: Deferred tax liability, net	0
Total net assets acquired	27,725
Contingent consideration liability				$ 2,500
Business acquisitions	25,200
LaSalle Bristol [Member]
Business Acquisition [Line Items]
Trade receivables	8,249
Inventories	46,017
Property, plant and equipment	8,500
Prepaid expenses & other	6,255
Intangible assets	5,885
Goodwill	3,718
Less: Total liabilities	28,594
Less: Deferred tax liability, net	41
Total net assets acquired	49,989
Medallion [Member]
Business Acquisition [Line Items]
Trade receivables		2,233
Inventories		2,605
Property, plant and equipment		1,713
Prepaid expenses & other		118
Intangible assets		3,100
Goodwill		1,342
Less: Total liabilities		1,200
Less: Deferred tax liability, net		0
Total net assets acquired		9,911
LPE [Member]
Business Acquisition [Line Items]
Trade receivables		5,848
Inventories		5,162
Property, plant and equipment		9,225
Prepaid expenses & other		337
Intangible assets		33,275
Goodwill		39,945
Less: Total liabilities		6,358
Less: Deferred tax liability, net		14,140
Total net assets acquired		73,294
Wire Design [Member]
Business Acquisition [Line Items]
Trade receivables		615
Inventories		437
Property, plant and equipment		555
Prepaid expenses & other		21
Intangible assets		5,590
Goodwill		4,052
Less: Total liabilities		491
Less: Deferred tax liability, net		0
Total net assets acquired		10,779
Baymont [Member]
Business Acquisition [Line Items]
Trade receivables		0
Inventories		1,174
Property, plant and equipment		2,067
Prepaid expenses & other		0
Intangible assets		3,166
Goodwill		1,502
Less: Total liabilities		69
Less: Deferred tax liability, net		0
Total net assets acquired		7,840
Contingent consideration liability	4,000			$ 4,000			$ 5,100
Business acquisitions		3,800
Indiana Transport [Member]
Business Acquisition [Line Items]
Trade receivables		6,379
Inventories		0
Property, plant and equipment		2,594
Prepaid expenses & other		1,309
Intangible assets		31,675
Goodwill		19,950
Less: Total liabilities		3,117
Less: Deferred tax liability, net		0
Total net assets acquired		58,790
LMI [Member]
Business Acquisition [Line Items]
Trade receivables		11,205
Inventories		9,071
Property, plant and equipment		6,028
Prepaid expenses & other		449
Intangible assets		32,810
Goodwill		29,241
Less: Total liabilities		8,471
Less: Deferred tax liability, net		0
Total net assets acquired		80,333
Nickell [Member]
Business Acquisition [Line Items]
Trade receivables		1,784
Inventories		1,547
Property, plant and equipment		1,240
Prepaid expenses & other		0
Intangible assets		6,250
Goodwill		2,331
Less: Total liabilities		556
Less: Deferred tax liability, net		0
Total net assets acquired		12,596
Other [Member]
Business Acquisition [Line Items]
Trade receivables	472	0
Inventories	329	250
Property, plant and equipment	300	2,668
Prepaid expenses & other	13	0
Intangible assets	1,667	0
Goodwill	902	668
Less: Total liabilities	196	124
Less: Deferred tax liability, net	0	0
Total net assets acquired	3,487	3,462
Parkland [Member]
Business Acquisition [Line Items]
Trade receivables			2,880
Inventories			5,280
Property, plant and equipment			2,987
Prepaid expenses & other			86
Intangible assets			10,950
Goodwill			5,175
Less: Total liabilities			2,180
Less: Deferred tax liability, net			0
Total net assets acquired			25,178
Progressive [Member]
Business Acquisition [Line Items]
Trade receivables			996
Inventories			3,074
Property, plant and equipment			100
Prepaid expenses & other			61
Intangible assets			6,010
Goodwill			2,980
Less: Total liabilities			2,344
Less: Deferred tax liability, net			0
Total net assets acquired			10,877
Cana [Member]
Business Acquisition [Line Items]
Trade receivables			646
Inventories			1,151
Property, plant and equipment			5,840
Prepaid expenses & other			29
Intangible assets			7,065
Goodwill			2,927
Less: Total liabilities			1,135
Less: Deferred tax liability, net			0
Total net assets acquired			16,523
MSM [Member]
Business Acquisition [Line Items]
Trade receivables			2,017
Inventories			1,592
Property, plant and equipment			2,521
Prepaid expenses & other			12
Intangible assets			7,855
Goodwill			984
Less: Total liabilities			965
Less: Deferred tax liability, net			0
Total net assets acquired			14,016
LS Mfg [Member]
Business Acquisition [Line Items]
Trade receivables			620
Inventories			1,382
Property, plant and equipment			265
Prepaid expenses & other			0
Intangible assets			6,315
Goodwill			2,772
Less: Total liabilities			154
Less: Deferred tax liability, net			0
Total net assets acquired			11,200
BHE [Member]
Business Acquisition [Line Items]
Trade receivables			2,922
Inventories			3,801
Property, plant and equipment			1,794
Prepaid expenses & other			0
Intangible assets			21,140
Goodwill			15,716
Less: Total liabilities			1,508
Less: Deferred tax liability, net			8,865
Total net assets acquired			35,000
Sigma/KRA [Member]
Business Acquisition [Line Items]
Trade receivables			2,039
Inventories			1,820
Property, plant and equipment			935
Prepaid expenses & other			7
Intangible assets			13,495
Goodwill			9,533
Less: Total liabilities			1,708
Less: Deferred tax liability, net			0
Total net assets acquired			26,121
Acquired Entities [Member]
Business Acquisition [Line Items]
Trade receivables	31,419	28,064	12,120
Inventories	98,276	20,246	18,100
Property, plant and equipment	45,802	26,090	14,442
Prepaid expenses & other	8,057	2,234	195
Intangible assets	154,564	115,866	72,830
Goodwill	73,842	99,031	40,087
Less: Total liabilities	50,232	20,386	9,994
Less: Deferred tax liability, net	8,880	14,140	8,865
Total net assets acquired	$ 352,848	$ 257,005	$ 138,915