Concentration of Credit Risk - Trade Receivable Risk (Details) - USD ($) $ in Thousands	3 Months Ended										6 Months Ended		12 Months Ended
	Jan. 26, 2019	Oct. 27, 2018	Jul. 28, 2018	Apr. 28, 2018	Jan. 27, 2018	Oct. 28, 2017	Jul. 29, 2017	Apr. 29, 2017	Jan. 28, 2017	Oct. 29, 2016	Jan. 27, 2018	Jan. 28, 2017	Jan. 26, 2019	Jul. 29, 2017	Jul. 30, 2016
Concentration Risk [Line Items]
Revenues	$ 748,619	$ 848,237	$ 799,470	$ 731,375	$ 655,133	$ 756,215	$ 780,188	$ 786,338	$ 701,131	$ 799,223	$ 1,411,348	$ 1,500,355	$ 3,127,700	$ 3,066,880	$ 2,672,542
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%
Customer Concentration Risk | Sales Revenue, Services, Net | AT&T Inc.
Concentration Risk [Line Items]
Revenues											$ 304,400		$ 650,200	$ 543,600	$ 363,100
Concentration risk percentage											21.60%		20.80%	17.70%	13.60%
Customer Concentration Risk | Sales Revenue, Services, Net | Verizon Communications Inc.(1)
Concentration Risk [Line Items]
Revenues											$ 247,000		$ 425,600	$ 556,800	$ 394,000
Concentration risk percentage											17.50%		13.60%	18.20%	14.70%
Customer Concentration Risk | Sales Revenue, Services, Net | Verizon Communications Inc
Concentration Risk [Line Items]
Revenues											$ 168,700		$ 599,800	$ 282,700	$ 298,200
Concentration risk percentage											12.00%		19.20%	9.20%	11.20%
Customer Concentration Risk | Sales Revenue, Services, Net | Comcast Corporation
Concentration Risk [Line Items]
Revenues											$ 290,100		$ 664,200	$ 806,700	$ 650,900
Concentration risk percentage											20.60%		21.20%	26.30%	24.40%
Customer Concentration Risk | Sales Revenue, Services, Net | Windstream Corporation
Concentration Risk [Line Items]
Revenues													$ 113,600
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | AT&T Inc.
Concentration Risk [Line Items]
Concentration risk percentage											24.50%		15.40%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | Verizon Communications Inc.(1)
Concentration Risk [Line Items]
Concentration risk percentage											18.50%		17.90%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | Verizon Communications Inc
Concentration Risk [Line Items]
Concentration risk percentage											14.40%		36.20%
Customer Concentration Risk | Trade Accounts Receivable and Costs and Estimated Earnings | Comcast Corporation
Concentration Risk [Line Items]
Concentration risk percentage											11.60%		11.00%
Telecommunications [Member] | Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Revenues											$ 1,284,100		$ 2,855,800	$ 2,819,900	$ 2,424,200
Concentration risk percentage											91.00%		91.30%	91.90%	90.70%
Underground Facility Locating [Member] | Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Revenues											$ 88,600		$ 182,700	$ 167,900	$ 156,700
Concentration risk percentage											6.30%		5.80%	5.50%	5.90%
Electrical and Gas Utilities and Other [Member] | Customer Concentration Risk | Sales Revenue, Services, Net
Concentration Risk [Line Items]
Revenues											$ 38,600		$ 89,200	$ 79,100	$ 91,600
Concentration risk percentage											2.70%		2.90%	2.60%	3.40%