Guarantees (Details) (USD $)	3 Months Ended		6 Months Ended
	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 8,800,000		$ 8,800,000
Advanced Filtration Systems, Inc. [Member]
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	28,600,000		28,600,000
Joint venture investment earnings	700,000	0	1,300,000	700,000
Royalty income	$ 1,800,000	$ 1,300,000	$ 3,300,000	$ 3,000,000