Concentration of Credit Risk (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jan. 23, 2016 USD ($) customer	Jul. 25, 2015 USD ($)	Jan. 24, 2015	Jan. 23, 2016 USD ($) customer	Jan. 24, 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	66.70%		59.80%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	22.40%		22.00%	20.60%	21.60%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink, Inc.
Concentration Risk [Line Items]
Concentration risk percentage	14.90%		14.00%	15.30%	13.50%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast Corporation
Concentration Risk [Line Items]
Concentration risk percentage	13.50%		13.10%	12.70%	12.90%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon [Member]
Concentration Risk [Line Items]
Concentration risk percentage	11.90%		6.30%	10.70%	6.90%
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage	19.30%	17.70%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 117.1	$ 101.7		$ 117.1
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | CenturyLink, Inc.
Concentration Risk [Line Items]
Concentration risk percentage	12.80%	14.00%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 77.9	$ 80.1		77.9
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Comcast Corporation
Concentration Risk [Line Items]
Concentration risk percentage	11.50%	11.00%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 70.1	$ 63.0		70.1
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	$ 64.0	$ 47.8		64.0
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Another Customer
Concentration Risk [Line Items]
Concentration risk percentage	13.30%	11.20%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 80.9	$ 64.5		80.9
Trade Accounts Receivable and Costs and Estimated Earnings | Customer Concentration Risk | Verizon [Member]
Concentration Risk [Line Items]
Concentration risk percentage	10.70%	8.90%
Accounts receivable, costs, and estimated earnings in excess of billings	$ 65.2	$ 50.8		$ 65.2