Accounts Receivable (Details) - USD ($)			9 Months Ended
	Apr. 30, 2020	Jul. 31, 2019	Apr. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 139,717,000	$ 146,899,000	$ 139,717,000
Less allowance for doubtful accounts	1,830,000	1,867,000	1,830,000
Accounts receivable, net	$ 137,887,000	$ 145,032,000	137,887,000
U.S. government | Accounts Receivable | Customer concentration risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	32.10%	27.80%
Verizon | Accounts Receivable | Customer concentration risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	10.20%
Billed Receivables | Commercial and International
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 76,909,000	$ 85,556,000	76,909,000
Billed Receivables | U.S. government
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	40,687,000	38,856,000	$ 40,687,000
Unbilled Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
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	17,919,000	20,469,000	$ 17,919,000
Unbilled Receivables | U.S. government
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 4,202,000	$ 2,018,000	$ 4,202,000