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Concentration of Credit Risk - Revenue Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 27, 2019	Jul. 28, 2018	Jul. 27, 2019	Jul. 28, 2018
Concentration Risk
Contract revenues	$ 884,221	$ 799,470	$ 1,717,964	$ 1,530,844
Concentration risk percentage	100.00%	100.00%	100.00%	100.00%
Sales Revenue, Services, Net | Customer Concentration Risk
Concentration Risk
Concentration risk percentage			10.00%
Sales Revenue, Services, Net | Customer Concentration Risk | Five Unnamed Customers
Concentration Risk
Concentration risk percentage			79.40%	78.30%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk
Contract revenues	$ 183,300	$ 165,200	$ 392,500	$ 342,200
Concentration risk percentage	20.70%	20.70%	22.80%	22.40%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon Communications Inc.
Concentration Risk
Contract revenues	$ 133,200	$ 171,200	$ 270,300	$ 330,300
Concentration risk percentage	15.10%	21.40%	15.70%	21.60%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink, Inc.
Concentration Risk
Contract revenues	$ 205,000	$ 147,300	$ 384,800	$ 269,500
Concentration risk percentage	23.20%	18.40%	22.40%	17.60%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast Corporation
Concentration Risk
Contract revenues	$ 138,700	$ 107,600	$ 248,500	$ 197,200
Concentration risk percentage	15.70%	13.50%	14.50%	12.90%
Sales Revenue, Services, Net | Customer Concentration Risk | Total other customers combined
Concentration Risk
Contract revenues	$ 224,000	$ 208,200	$ 421,900	$ 391,600
Concentration risk percentage	25.30%	26.00%	24.60%	25.50%

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