QUARTERLY FINANCIAL INFORMATION (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
QUARTERLY FINANCIAL INFORMATION (Unaudited) [Abstract]
Sales	$ 313,800	$ 329,700	$ 307,900	$ 290,100	$ 293,000	$ 289,900	$ 261,900	$ 252,300	$ 218,300	$ 207,900	$ 197,700	$ 183,100	$ 1,241,510	$ 1,097,110	$ 807,005
Gross profit	94,600	97,100	91,500	89,100	87,500	83,500	76,600	71,500	62,600	59,500	57,300	52,400	372,345	319,091	231,836
Net income	$ 16,900	$ 16,400	$ 13,700	$ 13,200	$ 14,100	$ 13,100	$ 12,200	$ 11,600	$ 9,200	$ 8,300	$ 7,600	$ 6,300	$ 60,237	$ 50,985	$ 31,437
Earnings per share - basic (in dollars per share)	$ 1.17	$ 1.13	$ 0.95	$ 0.92	$ 0.98	$ 0.91	$ 0.84	$ 0.81	$ 0.64	$ 0.58	$ 0.53	$ 0.44	$ 4.17	$ 3.54	$ 2.19
Earnings per share - diluted (in dollars per share)	$ 1.10	$ 1.07	$ 0.90	$ 0.87	$ 0.92	$ 0.86	$ 0.80	$ 0.77	$ 0.61	$ 0.55	$ 0.50	$ 0.42	$ 3.94	$ 3.35	$ 2.08