Accounts Receivable (Details)	Oct. 31, 2017 USD ($) customer	Jul. 31, 2017 USD ($) customer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 115,445,000	$ 126,262,000
Less allowance for doubtful accounts	1,502,000	1,300,000
Accounts receivable, net	$ 113,943,000	$ 124,962,000
Accounts Receivable | Major customer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of customers accounted for greater than 10% of total accounts receivable other than disclosed | customer	0	0
Minimum percentage of total accounts receivable outstanding by one customer or individual country requiring additional and separate disclosure	10.00%	10.00%
Accounts Receivable | Major customer | U.S. government and its agencies
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total accounts receivable	27.60%	23.90%
Billed Receivables | Commercial and International
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 66,349,000	$ 71,404,000
Billed Receivables | U.S. government and its agencies
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	14,709,000	18,497,000
Unbilled Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retainage	$ 108,000	118,000
Maximum period that management estimates unbilled accounts receivable to be billed and collected	1 year
Unbilled Receivables | Commercial and International
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 17,215,000	24,668,000
Unbilled Receivables | U.S. government and its agencies
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	17,172,000	11,693,000
Unbilled Receivables | Major customer | U.S. Prime Contractor
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unbilled receivable related to our large over-the-horizon microwave system contracts	$ 2,839,000	$ 2,995,000