Customer Concentration and Revenue Information - Trade Receivable 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%
Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Concentration risk percentage											10.00%
Telecommunications | Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Revenues									$ 1,284,100		$ 3,031,900	$ 2,855,800	$ 2,819,900
Concentration risk percentage									91.00%		90.80%	91.30%	91.90%
Underground facility locating | Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Revenues									$ 88,600		$ 204,500	$ 182,700	$ 167,900
Concentration risk percentage									6.30%		6.10%	5.80%	5.50%
Electrical and gas utilities and other | Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Revenues									$ 38,600		$ 103,300	$ 89,200	$ 79,100
Concentration risk percentage									2.70%		3.10%	2.90%	2.60%