Customer Concentration and Revenue Information - Revenue Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended		12 Months Ended
	Jan. 25, 2020	Oct. 26, 2019	Jul. 27, 2019	Apr. 27, 2019	Jan. 26, 2019	Oct. 27, 2018	Jul. 28, 2018	Apr. 28, 2018	Jan. 27, 2018	Jan. 28, 2017	Jan. 25, 2020	Jan. 26, 2019	Jul. 29, 2017
Concentration Risk [Line Items]
Revenues	$ 737,603	$ 884,115	$ 884,221	$ 833,743	$ 748,619	$ 848,237	$ 799,470	$ 731,375	$ 1,411,348	$ 1,500,355	$ 3,339,682	$ 3,127,700	$ 3,066,880
Concentration risk percentage									100.00%		100.00%	100.00%	100.00%
Sales Revenue, Services, Net | Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk percentage											10.00%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon Communications Inc
Concentration Risk [Line Items]
Revenues									$ 168,700		$ 728,200	$ 599,800	$ 282,700
Concentration risk percentage									12.00%		21.80%	19.20%	9.20%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk [Line Items]
Revenues									$ 290,100		$ 687,900	$ 664,200	$ 806,700
Concentration risk percentage									20.60%		20.60%	21.20%	26.30%
Sales Revenue, Services, Net | Customer Concentration Risk | Century Link, Inc
Concentration Risk [Line Items]
Revenues									$ 247,000		$ 547,800	$ 425,600	$ 556,800
Concentration risk percentage									17.50%		16.40%	13.60%	18.20%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast Corporation
Concentration Risk [Line Items]
Revenues									$ 304,400		$ 503,200	$ 650,200	$ 543,600
Concentration risk percentage									21.60%		15.10%	20.80%	17.70%
Sales Revenue, Services, Net | Customer Concentration Risk | Total other customers combined
Concentration Risk [Line Items]
Revenues									$ 401,100		$ 872,600	$ 787,900	$ 877,100
Concentration risk percentage									28.30%		26.10%	25.20%	28.60%