Unaudited Quarterly Financial Information (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
Net sales	$ 297,499	$ 298,421	$ 287,780	$ 290,151	$ 289,212	$ 275,927	$ 268,001	$ 280,176	$ 1,173,851	$ 1,113,316	$ 1,120,625
Gross margin	147,452	151,082	143,692	146,065	143,867	139,909	134,158	140,358	588,291	558,292	558,773
Operating income	44,780	37,709	34,796	35,411	36,295	31,550	29,962	33,208	152,696	131,015	117,878
Net earnings	$ 34,951	$ 26,000	$ 4,273	$ 25,836	$ 25,242	$ 22,553	$ 25,297	$ 22,553	$ 91,060	$ 95,645	$ 80,110
Class A Nonvoting Common Stock [Member]
Net earnings (loss) from continuing operations per Class A Common Share:
Earnings (loss) from continuing operations, per basic share	$ 0.67	$ 0.50	$ 0.08	$ 0.50	$ 0.49	$ 0.44	$ 0.50	$ 0.45	$ 1.76	$ 1.87
Diluted (loss) earnings from continuing operations per Class A Common Share, as reported	$ 0.66	$ 0.49	$ 0.08	$ 0.49	$ 0.48	$ 0.43	$ 0.49	$ 0.44	$ 1.73	$ 1.84