SEGMENT REPORTING, Business Segmented Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Sales	$ 251,930	$ 230,025	$ 741,155	$ 739,801
Income (loss) from operations	6,510	4,933	25,003	11,780
Reportable Segment [Member]
Segment Reporting Information [Line Items]
Sales	251,930	230,025	741,155	739,801
Amortization	4,336	4,519	12,943	13,557
Income (loss) from operations	17,034	14,385	53,226	40,956
Operating income, excluding amortization	21,370	18,904	66,169	54,513
Reportable Segment [Member] | Service Centers [Member]
Segment Reporting Information [Line Items]
Sales	160,863	152,018	474,324	481,352
Amortization	2,262	2,292	6,738	6,871
Income (loss) from operations	13,288	11,053	40,570	28,608
Operating income, excluding amortization	15,550	13,345	47,308	35,479
Reportable Segment [Member] | Innovative Pumping Solutions [Member]
Segment Reporting Information [Line Items]
Sales	51,027	39,830	144,555	141,614
Amortization	1,803	1,956	5,393	5,874
Income (loss) from operations	35	(326)	1,710	1,549
Operating income, excluding amortization	1,838	1,630	7,103	7,423
Reportable Segment [Member] | Supply Chain Services [Member]
Segment Reporting Information [Line Items]
Sales	40,040	38,177	122,276	116,835
Amortization	271	271	812	812
Income (loss) from operations	3,711	3,658	10,946	10,799
Operating income, excluding amortization	$ 3,982	$ 3,929	$ 11,758	$ 11,611