Guarantees (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017	Jul. 31, 2017
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 8,400,000		$ 8,400,000		$ 10,500,000
Amount drawn upon letters of credit	0		0		$ 0
Advanced Filtration Systems, Inc.
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	31,200,000		31,200,000
Joint venture investment earnings (loss)	400,000	$ 300,000	500,000	$ 900,000
Royalty income	$ 1,700,000	$ 1,400,000	$ 3,600,000	$ 2,700,000