Concentration of Credit Risk (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jan. 28, 2017 USD ($) customer	Jul. 30, 2016 USD ($)	Jan. 23, 2016	Jan. 28, 2017 USD ($) customer	Jan. 23, 2016
Concentration Risk [Line Items]
Number of customers classified as highly concentrated | customer	5			5
Concentration risk percentage	16.70%		13.50%
Sales Revenue, Services, Net | Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk percentage				10.00%
Sales Revenue, Services, Net | Customer Concentration Risk | Five Unnamed Customers
Concentration Risk [Line Items]
Concentration risk percentage				75.50%	66.70%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	28.30%		22.40%	28.70%	20.60%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast
Concentration Risk [Line Items]
Concentration risk percentage				15.80%	12.70%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink
Concentration Risk [Line Items]
Concentration risk percentage	16.50%		14.90%	16.10%	15.30%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon
Concentration Risk [Line Items]
Concentration risk percentage	8.80%		11.90%	9.10%	10.70%
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk percentage	10.00%
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	17.40%	20.30%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 118.9	$ 138.8		$ 118.9
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Comcast
Concentration Risk [Line Items]
Concentration risk percentage	18.50%	13.90%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 126.4	$ 95.3		126.4
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | CenturyLink
Concentration Risk [Line Items]
Concentration risk percentage	11.70%	11.50%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 79.7	$ 79.0		79.7
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Verizon
Concentration Risk [Line Items]
Concentration risk percentage	9.30%	10.10%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 63.2	$ 69.0		63.2
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Windstream Corporation
Concentration Risk [Line Items]
Concentration risk percentage	14.30%	11.50%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 97.6	$ 79.0		97.6
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Another Customer
Concentration Risk [Line Items]
Concentration risk percentage	11.10%	10.40%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 75.6	$ 71.5		$ 75.6