SEGMENT REPORTING - Financial Information of Company's Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 285,691	$ 251,401	$ 531,278	$ 552,384
Income from operations	34,591	25,968	59,998	57,480
Product sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	220,557	177,274	417,676	391,221
Inventory services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,263	4,086	8,459	8,311
Staffing services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	24,144	9,562	45,171	22,352
Pump production and delivery | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	36,727	60,479	59,972	130,500
SC
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	209,633	153,848	396,002	336,433
Income from operations	26,300	14,050	48,437	31,379
SC | Product sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	185,489	144,286	350,831	314,081
SC | Inventory services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
SC | Staffing services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	24,144	9,562	45,171	22,352
SC | Pump production and delivery | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	36,727	60,479	59,972	130,500
Income from operations	4,803	8,565	5,751	18,993
IPS | Product sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Inventory services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Staffing services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Pump production and delivery | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	36,727	60,479	59,972	130,500
SCS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	39,331	37,074	75,304	85,451
Income from operations	3,488	3,353	5,810	7,108
SCS | Product sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	35,068	32,988	66,845	77,140
SCS | Inventory services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,263	4,086	8,459	8,311
SCS | Staffing services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
SCS | Pump production and delivery | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 0	$ 0	$ 0	$ 0