Guarantees (Details) - USD ($)	3 Months Ended		9 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2018	Apr. 30, 2017	Jul. 31, 2017
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 8,200,000		$ 8,200,000		$ 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	33,500,000		33,500,000
Joint venture investment earnings (loss)	600,000	$ 700,000	1,100,000	$ 1,600,000
Royalty income	$ 1,700,000	$ 1,600,000	$ 5,300,000	$ 4,300,000