Guarantees (Details) (USD $)	3 Months Ended		6 Months Ended
	Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2015	Jan. 31, 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	23,100,000		23,100,000
Joint venture investment earnings	600,000	700,000	1,500,000	1,300,000
Royalty income	$ 1,500,000	$ 1,800,000	$ 3,100,000	$ 3,300,000