SEGMENT AND GEOGRAPHICAL REPORTING - Financial Information of Company's Segments (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
Segment Reporting Information [Line Items]
Sales	$ 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
Operating income for reportable segments, excluding amortization													130,865	90,124	72,950
Identifiable assets at year end	645,226				618,224				590,255				645,226	618,224	590,255
Capital expenditures													8,751	2,646	4,403
Proceeds from sale of fixed assets													12		1,206
Depreciation													8,087	9,463	10,480
Amortization													16,586	17,266	18,061
Interest expense													20,937	17,054	15,564
Adjustments for:
Amortization of intangible assets													16,586	17,266	18,061
Total operating income													68,451	33,490	19,332
Other income, net													(1,192)	(456)	(5,906)
Income (loss) before income taxes													48,706	16,892	9,674
Service Centers
Segment Reporting Information [Line Items]
Sales													750,044	641,275	621,007
Innovative Pumping Solutions
Segment Reporting Information [Line Items]
Sales													291,697	204,030	187,124
Supply Chain Services
Segment Reporting Information [Line Items]
Sales													174,456	161,477	153,961
Operating Segments
Segment Reporting Information [Line Items]
Operating income for reportable segments, excluding amortization													130,865	90,124	72,950
Operating Segments | Service Centers
Segment Reporting Information [Line Items]
Sales													750,044	641,275	621,007
Operating income for reportable segments, excluding amortization													80,718	63,250	47,634
Identifiable assets at year end	402,944				385,744				370,261				402,944	385,744	370,261
Capital expenditures													1,655	1,076	447
Proceeds from sale of fixed assets													3		1,038
Depreciation													3,974	5,162	6,520
Amortization													9,272	8,989	9,152
Interest expense													11,178	9,712	9,290
Operating Segments | Innovative Pumping Solutions
Segment Reporting Information [Line Items]
Sales													291,697	204,030	187,124
Operating income for reportable segments, excluding amortization													33,943	11,423	9,867
Identifiable assets at year end	188,765				172,538				175,198				188,765	172,538	175,198
Capital expenditures													6,800	1,488	3,827
Proceeds from sale of fixed assets													9		168
Depreciation													4,064	4,198	3,834
Amortization													6,237	7,194	7,826
Interest expense													7,351	5,352	4,422
Operating Segments | Supply Chain Services
Segment Reporting Information [Line Items]
Sales													174,456	161,477	153,961
Operating income for reportable segments, excluding amortization													16,204	15,451	15,449
Identifiable assets at year end	$ 53,517				$ 59,942				$ 44,796				53,517	59,942	44,796
Capital expenditures													296	82	129
Proceeds from sale of fixed assets													0		0
Depreciation													49	103	126
Amortization													1,077	1,083	1,083
Interest expense													2,408	1,990	1,852
Segment Reconciling Items
Adjustments for:
Amortization of intangible assets													16,586	17,266	18,061
Corporate and other expense, net													$ 45,828	$ 39,368	$ 35,557