SEGMENT REPORTING - Financial Information of Company's Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 251,401	$ 333,318	$ 552,384	$ 644,543
Income from operations(1)	25,582	39,042	56,691	68,907
Product Sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	177,274	232,829	391,221	450,882
Inventory Management Services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,086	3,927	8,311	7,865
Staffing Services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	9,562	15,534	22,352	30,045
Customized Pump Production | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	60,479	81,028	130,500	155,751
SC
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	153,848	199,978	336,433	386,157
Income from operations(1)	13,664	23,230	30,590	42,210
SC | Product Sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	144,286	184,444	314,081	356,112
SC | Inventory Management 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	9,562	15,534	22,352	30,045
SC | Customized Pump Production | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	60,479	81,028	130,500	155,751
Income from operations(1)	8,565	12,028	18,993	18,827
IPS | Product Sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Inventory Management 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 | Customized Pump Production | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	60,479	81,028	130,500	155,751
SCS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	37,074	52,312	85,451	102,635
Income from operations(1)	3,353	3,784	7,108	7,870
SCS | Product Sales | Recognized at a point in time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	32,988	48,385	77,140	94,770
SCS | Inventory Management Services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,086	3,927	8,311	7,865
SCS | Staffing Services | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
SCS | Customized Pump Production | Recognized over time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 0	$ 0	$ 0	$ 0