Concentration of Credit Risk - Trade Receivable Risk (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jan. 27, 2018	Jul. 29, 2017	Jul. 30, 2016
Concentration Risk [Line Items]
Accounts receivable, net	$ 318,684	$ 369,800	$ 328,030
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings
Concentration Risk [Line Items]
Concentration risk percentage	10.00%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | Comcast Corporation
Concentration Risk [Line Items]
Accounts receivable, net	$ 166,500	$ 159,700	$ 95,300
Concentration risk percentage	24.50%	21.30%	13.90%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | CenturyLink, Inc.(1)
Concentration Risk [Line Items]
Accounts receivable, net	$ 126,000	$ 148,100	$ 81,500
Concentration risk percentage	18.50%	19.80%	11.90%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | Verizon Communications Inc
Concentration Risk [Line Items]
Accounts receivable, net	$ 98,200	$ 73,700	$ 70,200
Concentration risk percentage	14.40%	9.80%	10.20%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | AT&T Inc.
Concentration Risk [Line Items]
Accounts receivable, net	$ 79,200	$ 87,100	$ 138,800
Concentration risk percentage	11.60%	11.60%	20.30%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | Windstream Corporation
Concentration Risk [Line Items]
Accounts receivable, net	$ 42,900	$ 84,700	$ 79,000
Concentration risk percentage	6.30%	11.30%	11.50%