SEGMENT REPORTING - Financial Information of Company's Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 300,983	$ 311,225
Income from operations	31,109	29,865
Product Sales | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	213,947	218,053
Inventory Management Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,225	3,938
Staffing Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	12,790	14,511
Customized Pump Production | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	70,021	74,723
SC
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	182,585	186,179
Income from operations	16,926	18,980
SC | Product Sales | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	169,795	171,668
SC | Inventory Management Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
SC | Staffing Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	12,790	14,511
SC | Customized Pump Production | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	70,021	74,723
Income from operations	10,428	6,799
IPS | Product Sales | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS | Inventory Management Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS | Staffing Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS | Customized Pump Production | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	70,021	74,723
SCS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	48,377	50,323
Income from operations	3,755	4,086
SCS | Product Sales | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	44,152	46,385
SCS | Inventory Management Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,225	3,938
SCS | Staffing Services | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
SCS | Customized Pump Production | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 0	$ 0