QUARTERLY FINANCIAL RESULTS (UNAUDITED) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Results (Unaudited).
Net sales	$ 735,548	$ 708,485	$ 676,165	$ 572,394	$ 684,323	$ 680,769	$ 627,622	$ 581,259	$ 2,692,592	$ 2,573,973	$ 2,463,339
Cost of sales	637,425	611,518	582,291	490,201	592,472	592,201	544,635	503,646	2,321,435	2,232,954	2,139,950
Gross profit	98,123	96,967	93,874	82,193	91,851	88,568	82,987	77,613	371,157	341,019	323,389
Selling, general and administrative expenses	76,222	74,522	72,864	67,029	68,960	66,707	63,364	63,434	290,637	262,465	251,935
Income from operations	21,901	22,445	21,010	15,164	22,891	21,861	19,623	14,179	80,520	78,554	71,454
Interest expense	(14)	(27)	(12)	(14)	(20)	(29)	(39)	1	(67)	(87)	(86)
Income before taxes	21,887	22,418	20,998	15,150	22,871	21,832	19,584	14,180	80,453	78,467	71,368
Income tax provision	(8,890)	(8,825)	(8,540)	(6,087)	(9,258)	(8,831)	(7,955)	(5,596)	(32,342)	(31,640)	(28,687)
Net income	$ 12,997	$ 13,593	$ 12,458	$ 9,063	$ 13,613	$ 13,001	$ 11,629	$ 8,584	$ 48,111	$ 46,827	$ 42,681
Earnings per common share:
Basic	$ 0.49	$ 0.51	$ 0.47	$ 0.34	$ 0.51	$ 0.49	$ 0.44	$ 0.33	$ 1.81	$ 1.77	$ 1.63
Diluted	$ 0.49	$ 0.51	$ 0.47	$ 0.34	$ 0.51	$ 0.49	$ 0.44	$ 0.32	$ 1.80	$ 1.76	$ 1.61
Denominator:
Basic	26,569	26,542	26,501	26,499	26,459	26,423	26,363	26,346	26,528	26,398	26,246
Diluted	26,738	26,736	26,691	26,671	26,632	26,622	26,616	26,593	26,719	26,616	26,512