SEGMENT REPORTING - Financial Information of Company's Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 333,318	$ 311,227	$ 644,543	$ 597,163
Income from operations	39,042	35,144	68,907	61,409
Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	232,829	216,318	450,882	416,564
Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	3,927	3,267	7,865	6,497
Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	15,534	17,385	30,045	32,203
Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	81,028	74,257	155,751	141,899
SC
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	199,978	193,576	386,157	368,937
Income from operations	23,230	21,933	42,210	37,762
SC | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	184,444	176,191	356,112	336,734
SC | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
SC | Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	15,534	17,385	30,045	32,203
SC | Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	81,028	74,257	155,751	141,899
Income from operations	12,028	8,956	18,827	15,338
IPS | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
IPS | Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	81,028	74,257	155,751	141,899
SCS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	52,312	43,394	102,635	86,327
Income from operations	3,784	4,255	7,870	8,309
SCS | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	48,385	40,127	94,770	79,830
SCS | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	3,927	3,267	7,865	6,497
SCS | Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	0	0	0	0
SCS | Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 0	$ 0	$ 0	$ 0