Intangible Assets and Goodwill - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jan. 31, 2018 USD ($)	Jan. 31, 2017 USD ($)	Jan. 31, 2018 USD ($) Unit Segment	Jan. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets	$ 7,600,000		$ 7,600,000		$ 7,600,000
Amortization expense related to acquired intangible assets	1,300,000	$ 2,000,000	2,600,000	$ 4,000,000
Goodwill	$ 2,344,000		$ 2,344,000		$ 2,344,000
Number of Reporting Units | Unit			2
Number of Reportable Segments | Segment			3
Security and Detection
Goodwill and Intangible Assets Disclosure [Line Items]
Impairment of goodwill			$ 0
Medical Imaging
Goodwill and Intangible Assets Disclosure [Line Items]
Impairment of goodwill			$ 0
Minimum
Goodwill and Intangible Assets Disclosure [Line Items]
Intangible assets, estimated useful lives			1 year
Maximum
Goodwill and Intangible Assets Disclosure [Line Items]
Intangible assets, estimated useful lives			14 years