SEGMENT REPORTING - Financial Information of Company's Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	$ 327,178	$ 308,028	$ 971,721	$ 905,191
Income from operations	38,278	33,249	107,185	94,658
Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	225,839	213,232	676,721	629,796
Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,284	3,393	12,149	9,890
Staffing Services | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	14,886	14,741	44,931	46,944
Customized Pump Production | Time-and-materials Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	82,169	76,662	237,920	218,561
SC
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	193,727	187,763	579,884	556,700
Income from operations	25,071	20,590	67,281	58,353
SC | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	178,841	173,022	534,953	509,756
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	14,886	14,741	44,931	46,944
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	82,169	76,662	237,920	218,561
Income from operations	10,097	8,773	28,924	24,109
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	82,169	76,662	237,920	218,561
SCS
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	51,282	43,603	153,917	129,930
Income from operations	3,110	3,886	10,980	12,196
SCS | Product Sales | Fixed-price Contract | Transferred at Point in Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	46,998	40,210	141,768	120,040
SCS | Inventory Management Services | Fixed-price Contract | Transferred over Time
Segment Reporting Information, Profit (Loss) [Abstract]
Total Revenue	4,284	3,393	12,149	9,890
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