Concentration of Credit Risk (Details) $ in Millions	3 Months Ended			9 Months Ended
	Apr. 23, 2016 USD ($) customer	Jul. 25, 2015 USD ($)	Apr. 25, 2015	Apr. 23, 2016 USD ($) customer	Apr. 25, 2015
Concentration Risk [Line Items]
Number of customers classified as highly concentrated | customer	5			5
Sales Revenue, Services, Net | Customer Concentration Risk | Five Unnamed Customers
Concentration Risk [Line Items]
Concentration risk percentage				68.80%	60.10%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	26.80%		21.30%	22.80%	21.50%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon [Member]
Concentration Risk [Line Items]
Concentration risk percentage	10.40%		7.50%	10.60%	7.10%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast [Member]
Concentration Risk [Line Items]
Concentration risk percentage	14.40%		13.30%	13.30%	13.00%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink [Member]
Concentration Risk [Line Items]
Concentration risk percentage	13.70%		13.90%	14.70%	13.70%
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	23.00%	17.70%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 161.9	$ 101.7		$ 161.9
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Windstream Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage	10.50%	8.30%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 73.9	$ 47.8		73.9
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Another Customer
Concentration Risk [Line Items]
Concentration risk percentage	12.00%	11.20%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 84.3	$ 64.5		84.3
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Verizon [Member]
Concentration Risk [Line Items]
Concentration risk percentage	10.20%	8.90%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 71.6	$ 50.8		71.6
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Comcast [Member]
Concentration Risk [Line Items]
Concentration risk percentage	13.10%	11.00%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 92.0	$ 63.0		92.0
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | CenturyLink [Member]
Concentration Risk [Line Items]
Concentration risk percentage	11.30%	14.00%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 79.4	$ 80.1		$ 79.4