Guarantees (Details) (USD $)	3 Months Ended		9 Months Ended
	Apr. 30, 2015	Apr. 30, 2014	Apr. 30, 2015	Apr. 30, 2014
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 7,900,000		$ 7,900,000
Amount drawn upon letters of credit	0		0
Advanced Filtration Systems, Inc. [Member]
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	27,800,000		27,800,000
Joint venture investment earnings	500,000	900,000	1,900,000	2,200,000
Royalty income	$ 1,500,000	$ 1,700,000	$ 4,600,000	$ 5,000,000