Quarterly Results of Operations (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2018	Jul. 31, 2017	Jul. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 228,854	$ 217,268	$ 213,034	$ 212,766	$ 205,502	$ 192,113	$ 184,817	$ 187,725	$ 871,922	$ 770,157	$ 664,755
Cost of sales	121,451	112,594	111,799	112,107	107,774	100,665	96,340	98,218	457,951	402,997	355,569
Gross profit	$ 107,403	$ 104,674	$ 101,235	$ 100,659	$ 97,728	$ 91,448	$ 88,477	$ 89,507	413,971	367,160	309,186
Gross profit percentage	46.90%	48.20%	47.50%	47.30%	47.60%	47.60%	47.90%	47.70%
Net income	$ 16,888	$ 18,736	$ 32,488	$ 22,929	$ 16,997	$ 17,511	$ 18,070	$ 18,800	$ 91,041	$ 71,378	$ 59,953
Earnings per common share:
Basic (in dollars per share)	$ 0.41	$ 0.45	$ 0.78	$ 0.55	$ 0.41	$ 0.42	$ 0.43	$ 0.45	$ 2.18	$ 1.71	$ 1.44
Diluted (in dollars per share)	$ 0.41	$ 0.45	$ 0.78	$ 0.55	$ 0.41	$ 0.42	$ 0.43	$ 0.45	$ 2.18	$ 1.71	$ 1.44