SEGMENT REPORTING, Business Segmented Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Sales	$ 250,698	$ 256,215	$ 489,225	$ 509,776
Income (loss) from operations	10,257	8,849	18,493	6,847
Reportable Segment [Member]
Segment Reporting Information [Line Items]
Sales	250,698	256,215	489,225	509,776
Amortization	4,291	4,510	8,607	9,038
Income (loss) from operations	19,599	17,776	36,191	26,571
Operating income, excluding amortization	23,890	22,286	44,798	35,609
Reportable Segment [Member] | Service Centers [Member]
Segment Reporting Information [Line Items]
Sales	164,749	161,832	313,461	329,334
Amortization	2,227	2,284	4,477	4,579
Income (loss) from operations	16,190	10,313	27,281	17,555
Operating income, excluding amortization	18,417	12,597	31,758	22,134
Reportable Segment [Member] | Innovative Pumping Solutions [Member]
Segment Reporting Information [Line Items]
Sales	44,470	54,353	93,528	101,784
Amortization	1,793	1,955	3,588	3,917
Income (loss) from operations	(38)	3,532	1,676	1,876
Operating income, excluding amortization	1,755	5,487	5,264	5,793
Reportable Segment [Member] | Supply Chain Services [Member]
Segment Reporting Information [Line Items]
Sales	41,479	40,030	82,236	78,658
Amortization	271	271	542	542
Income (loss) from operations	3,447	3,931	7,234	7,140
Operating income, excluding amortization	$ 3,718	$ 4,202	$ 7,776	$ 7,682