Concentration of Credit Risk (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		3 Months Ended		12 Months Ended
	Apr. 25, 2015	Apr. 26, 2014	Apr. 25, 2015	Apr. 26, 2014	Jul. 26, 2014
Concentration Risk [Line Items]
Number of customers classified as highly concentrated	5		5
Customer Concentration Risk | Five Unnamed Customers
Concentration Risk [Line Items]
Concentration risk percentage	60.10%	58.50%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	21.50%	18.90%	21.30%	20.80%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink, Inc.
Concentration Risk [Line Items]
Concentration risk percentage	13.70%	14.30%	13.90%	12.80%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast Corporation
Concentration Risk [Line Items]
Concentration risk percentage	13.00%	11.50%	13.30%	12.10%
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	20.70%				17.90%
Accounts receivable, costs, and estimated earnings in excess of billings	103.5		103.5		$ 87.6
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | CenturyLink, Inc.
Concentration Risk [Line Items]
Concentration risk percentage	11.10%				9.90%
Accounts receivable, costs, and estimated earnings in excess of billings	55.5		55.5		48.9
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Comcast Corporation
Concentration Risk [Line Items]
Concentration risk percentage	11.20%				9.80%
Accounts receivable, costs, and estimated earnings in excess of billings	56.2		56.2		48.2
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Another Customer
Concentration Risk [Line Items]
Concentration risk percentage	10.20%				4.70%
Accounts receivable, costs, and estimated earnings in excess of billings	51.0		51.0		$ 20.5