Quarterly Results of Operations (unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
	Jul. 31, 2013	Apr. 30, 2013	Jan. 31, 2013	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2013	Jul. 31, 2012	Jul. 31, 2011
Quarterly Results of Operations (unaudited)
Net sales	$ 113,973,000	$ 105,009,000	$ 106,363,000	$ 99,681,000	$ 98,693,000	$ 97,238,000	$ 97,297,000	$ 93,262,000	$ 425,026,000	$ 386,490,000	$ 321,651,000
Cost of sales	64,859,000	59,525,000	61,212,000	55,954,000	55,916,000	54,619,000	56,476,000	55,312,000	241,550,000	222,323,000	198,868,000
Gross profit	49,114,000	45,484,000	45,151,000	43,727,000	42,777,000	42,619,000	40,821,000	37,950,000	183,476,000	164,167,000	122,783,000
Gross profit percentage	43.10%	43.30%	42.40%	43.90%	43.30%	43.80%	42.00%	40.70%
Net income	10,213,000	8,998,000	10,452,000	9,576,000	9,649,000	8,174,000	7,294,000	6,220,000	39,239,000	31,337,000	20,425,000
Earnings per common share:
Basic (in dollars per share)	$ 0.25	$ 0.22	$ 0.26	$ 0.24	$ 0.24	$ 0.20	$ 0.18	$ 0.16	$ 0.96	$ 0.78	$ 0.53
Diluted (in dollars per share)	$ 0.25	$ 0.22	$ 0.25	$ 0.23	$ 0.24	$ 0.20	$ 0.18	$ 0.15	$ 0.95	$ 0.77	$ 0.52
Income tax benefit on investment losses					$ 1,000,000
Increase in basic and diluted earnings per share (in dollars per share)					$ 0.02