REVENUE RECOGNITION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenue	$ 311,000	$ 308,000	$ 311,200	$ 285,900	$ 265,700	$ 251,900	$ 250,700	$ 238,500	$ 222,300	$ 230,000	$ 256,200	$ 253,600	$ 1,216,197	$ 1,006,782	$ 962,092
Service Centers
Disaggregation of Revenue [Line Items]
Total revenue													750,044	641,275	621,007
Innovative Pumping Solutions
Disaggregation of Revenue [Line Items]
Total revenue													291,697	204,030	187,124
Supply Chain Services
Disaggregation of Revenue [Line Items]
Total revenue													174,456	161,477	153,961
Product sales | Transferred at Point in Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													846,079	723,255	705,176
Product sales | Service Centers | Transferred at Point in Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													685,309	575,328	565,157
Product sales | Innovative Pumping Solutions | Transferred at Point in Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													0	0	0
Product sales | Supply Chain Services | Transferred at Point in Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													160,770	147,927	140,019
Inventory management services | Transferred over Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													13,686	13,550	13,942
Inventory management services | Service Centers | Transferred over Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													0	0	0
Inventory management services | Innovative Pumping Solutions | Transferred over Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													0	0	0
Inventory management services | Supply Chain Services | Transferred over Time | Fixed-price Contract
Disaggregation of Revenue [Line Items]
Total revenue													13,686	13,550	13,942
Staffing services | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													64,735	65,947	55,850
Staffing services | Service Centers | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													64,735	65,947	55,850
Staffing services | Innovative Pumping Solutions | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													0	0	0
Staffing services | Supply Chain Services | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													0	0	0
Customized pump production | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													291,697	204,030	187,124
Customized pump production | Service Centers | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													0	0	0
Customized pump production | Innovative Pumping Solutions | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													291,697	204,030	187,124
Customized pump production | Supply Chain Services | Transferred over Time | Time-and-material Contract
Disaggregation of Revenue [Line Items]
Total revenue													$ 0	$ 0	$ 0