Accounts Receivable Concentration Credit Risk (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jan. 27, 2018	Jan. 26, 2019	Jul. 29, 2017	Jul. 30, 2016	Jan. 28, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Net, Current	$ 318,684	$ 625,258			$ 630,400
Concentration Risk, Percentage	100.00%	100.00%	100.00%	100.00%
Verizon [Member] | Trade Accounts Receivable and Costs and Estimated Earnings [Member] | Customer Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Net, Current	$ 98,200	$ 298,400
Concentration Risk, Percentage	14.40%	36.20%
Verizon Communications Inc.(1) | Trade Accounts Receivable and Costs and Estimated Earnings [Member] | Customer Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Net, Current	$ 126,000	$ 147,200
Concentration Risk, Percentage	18.50%	17.90%
Comcast [Member] | Trade Accounts Receivable and Costs and Estimated Earnings [Member] | Customer Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Net, Current	$ 166,500	$ 127,200
Concentration Risk, Percentage	24.50%	15.40%
ATT [Member] | Trade Accounts Receivable and Costs and Estimated Earnings [Member] | Customer Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts Receivable, Net, Current	$ 79,200	$ 90,600
Concentration Risk, Percentage	11.60%	11.00%