Guarantees (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016	Jul. 31, 2016
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 9,100,000		$ 9,100,000		$ 7,300,000
Amount drawn upon letters of credit	0		0
Advanced Filtration Systems, Inc.
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	23,000,000		23,000,000
Joint venture investment earnings (loss)	300,000	$ (100,000)	900,000	$ (1,200,000)
Royalty income	$ 1,400,000	$ 800,000	$ 2,700,000	$ 2,600,000