Guarantees (Details) - USD ($)	3 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Jul. 31, 2017
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 8,500,000		$ 10,500,000
Amount drawn upon letters of credit	0		$ 0
Advanced Filtration Systems, Inc.
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	29,200,000
Joint venture investment earnings (loss)	100,000	$ 600,000
Royalty income	$ 1,900,000	$ 1,300,000