QUARTERLY FINANCIAL DATA (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 176,034	$ 151,744	$ 152,390	$ 155,809	$ 151,590	$ 135,651	$ 137,489	$ 139,264	$ 635,977	$ 563,994	$ 544,753
Net income	$ 24,376	$ 25,591	$ 28,667	$ 28,714	$ 24,298	$ 26,227	$ 25,806	$ 28,010	$ 107,348	$ 104,341	$ 112,784
Net earnings per common share - basic	$ 0.25	$ 0.27	$ 0.30	$ 0.30	$ 0.25	$ 0.27	$ 0.27	$ 0.29	$ 1.12	$ 1.09	$ 1.19
Net earnings per common share - dilutive	$ 0.25	$ 0.26	$ 0.29	$ 0.30	$ 0.25	$ 0.27	$ 0.27	$ 0.29	$ 1.10	$ 1.08	$ 1.17
Cost of Goods Sold									$ 150,453	$ 134,705	$ 132,333
Gross Profit	$ 135,178	$ 114,946	$ 115,191	$ 120,209	$ 112,588	$ 104,198	$ 104,758	$ 107,745	$ 485,524	$ 429,289	$ 412,420