BUSINESS ACQUISITIONS - Narrative (Details)					12 Months Ended			36 Months Ended
	Dec. 31, 2020 USD ($)	Feb. 01, 2020 USD ($)	Jan. 01, 2020 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Cash consideration paid					$ 115,247,000	$ 0	$ 10,811,000
Acquired intangibles					39,797,000
Goodwill	$ 248,339,000				248,339,000	194,052,000	$ 194,052,000	$ 248,339,000
Service Centers
Business Acquisition [Line Items]
Goodwill	231,200,000				$ 231,200,000	160,934,000		231,200,000
Non-compete agreements
Business Acquisition [Line Items]
Amortization term of acquired intangibles					4 years 10 months 24 days
Customer relationships
Business Acquisition [Line Items]
Amortization term of acquired intangibles					7 years 3 months 18 days
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Contingent Consideration Liability
Business Acquisition [Line Items]
Fair value of contingent consideration liability	1,100,000				$ 1,100,000	$ 2,705,000		1,100,000
2020 Acquisitions
Business Acquisition [Line Items]
Cash consideration paid								115,247,000
Acquired intangibles					39,800,000
Goodwill	90,722,000				90,722,000			90,722,000
Fair value of accounts receivable acquired	20,204,000				20,204,000			20,204,000
Gross contractual amount of receivables acquired	21,100,000				21,100,000			21,100,000
Receivables acquired expected to be uncollectible	900,000				900,000			900,000
Acquisition related costs					172,000
Fair value of stock issued in acquisition								29,367,000
2020 Acquisitions | Service Centers
Business Acquisition [Line Items]
Goodwill	90,700,000				90,700,000			90,700,000
Goodwill expected to be deductible for tax purposes	0				0			0
2020 Acquisitions | Non-compete agreements
Business Acquisition [Line Items]
Acquired intangibles					$ 2,300,000
Amortization term of acquired intangibles					5 years
2020 Acquisitions | Customer relationships
Business Acquisition [Line Items]
Acquired intangibles					$ 37,500,000
Amortization term of acquired intangibles					8 years
2020 Acquisitions | Common Stock
Business Acquisition [Line Items]
Number of shares issued in acquisitions (in shares) | shares					1,480,909
TEC
Business Acquisition [Line Items]
Purchase price acquisition in cash and stock	64,700,000
APO
Business Acquisition [Line Items]
Purchase price acquisition in cash and stock	38,300,000
Pumping Solutions
Business Acquisition [Line Items]
Purchase price acquisition in cash and stock	21,000,000.0
CEC
Business Acquisition [Line Items]
Purchase price acquisition in cash and stock	4,500,000
PSI and Turbo
Business Acquisition [Line Items]
Percentage of revenue contributed by acquisitions					0.019
Percentage of net income (loss) contributed by acquisitions					(0.029)
Sales contributed by acquiree since acquisition					$ 19,600,000
Earnings before taxes contributed by acquiree since acquisition					800,000
Turbo
Business Acquisition [Line Items]
Cash consideration paid		$ 3,200,000
Additional goodwill recognized from acquisitions					0
PSI
Business Acquisition [Line Items]
Purchase price acquisition in cash and stock			$ 13,000,000.0
Additional goodwill recognized from acquisitions					463,000
ASI
Business Acquisition [Line Items]
Purchase price acquisition in cash and stock				$ 11,700,000
Cash consideration paid				10,800,000
Liability for contingent consideration				4,600,000
Fair value of stock issued in acquisition				900,000
Contingent consideration, maximum				$ 4,600,000
Payment period for contingent consideration				3 years
Contingent consideration liability, current	1,100,000				1,100,000			1,100,000
ASI | Fair Value, Inputs, Level 3
Business Acquisition [Line Items]
Contingent consideration, maximum				$ 4,000,000.0
ASI | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Contingent Consideration Liability
Business Acquisition [Line Items]
Fair value of contingent consideration liability	$ 1,100,000				$ 1,100,000			$ 1,100,000