Guarantees (Details) - USD ($)	3 Months Ended		9 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2017	Apr. 30, 2016	Jul. 31, 2016
Guarantor Obligations [Line Items]
Contingent liability for standby letters of credit, issued and outstanding	$ 9,100,000		$ 9,100,000		$ 7,300,000
Amount drawn upon letters of credit	0		0
Advanced Filtration Systems, Inc.
Guarantor Obligations [Line Items]
Outstanding debt of joint venture	28,600,000		28,600,000
Joint venture investment earnings (loss)	700,000	$ (200,000)	1,600,000	$ (1,000,000)
Royalty income	$ 1,600,000	$ 1,300,000	$ 4,300,000	$ 3,900,000