Concentration of Credit Risk (Details) $ in Millions	3 Months Ended		9 Months Ended
	Apr. 29, 2017 USD ($) customer	Apr. 23, 2016	Apr. 29, 2017 USD ($) customer	Apr. 23, 2016	Jul. 30, 2016 USD ($)
Concentration Risk [Line Items]
Number of customers classified as highly concentrated | customer	5		5
Concentration risk percentage	17.70%	13.70%
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			76.30%	68.80%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	27.10%	26.80%	28.10%	22.80%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast
Concentration Risk [Line Items]
Concentration risk percentage	19.40%	14.40%	17.10%	13.30%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink
Concentration Risk [Line Items]
Concentration risk percentage			16.60%	14.70%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon
Concentration Risk [Line Items]
Concentration risk percentage	8.50%	10.60%	8.90%	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			14.20%	20.30%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 109.5		$ 109.5		$ 138.8
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Comcast
Concentration Risk [Line Items]
Concentration risk percentage			22.40%	13.90%
Accounts receivable, costs, and estimated earnings in excess of billings	172.5		$ 172.5		95.3
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | CenturyLink
Concentration Risk [Line Items]
Concentration risk percentage			15.40%	11.50%
Accounts receivable, costs, and estimated earnings in excess of billings	118.4		$ 118.4		79.0
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Verizon
Concentration Risk [Line Items]
Concentration risk percentage			8.20%	10.20%
Accounts receivable, costs, and estimated earnings in excess of billings	63.0		$ 63.0		70.2
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Windstream Corporation
Concentration Risk [Line Items]
Concentration risk percentage			11.60%	11.50%
Accounts receivable, costs, and estimated earnings in excess of billings	89.5		$ 89.5		79.0
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Another Customer
Concentration Risk [Line Items]
Concentration risk percentage			9.30%	10.40%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 71.6		$ 71.6		$ 71.5