SEGMENT REPORTING - Financial Information of Company's Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 311,225	$ 285,936
Income from operations	29,865	26,266
Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	218,053	200,246
Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	3,938	3,230
Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	14,511	14,818
Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	74,723	67,642
SC
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	186,179	175,362
Income from operations	18,980	15,830
SC | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	171,668	160,544
SC | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
SC | Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	14,511	14,818
SC | Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	74,723	67,642
Income from operations	6,799	6,382
IPS | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS | Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
IPS | Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	74,723	67,642
SCS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	50,323	42,932
Income from operations	4,086	4,054
SCS | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	46,385	39,702
SCS | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	3,938	3,230
SCS | Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0
SCS | Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 0	$ 0