Guarantees (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2016	Jan. 31, 2015
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 7,800,000		$ 7,800,000
Amount drawn upon letters of credit	0		0
Advanced Filtration Systems, Inc.
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	20,300,000		20,300,000
Joint venture investment earnings (loss)	100,000	$ (600,000)	1,200,000	$ (1,500,000)
Royalty income	$ 800,000	$ 1,500,000	$ 2,600,000	$ 3,100,000