Concentration of Credit Risk - Revenue Concentration Risk (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 28, 2018	Jul. 29, 2017	Jul. 28, 2018	Jul. 29, 2017
Concentration Risk
Contract revenues	$ 799,470	$ 780,188	$ 1,530,844	$ 1,566,526
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			78.30%	77.60%
Sales Revenue, Services, Net | Customer Concentration Risk | AT&T Inc.
Concentration Risk
Contract revenues	$ 165,200	$ 163,500	$ 342,200	$ 376,600
Concentration risk percentage	20.70%	21.00%	22.40%	24.00%
Sales Revenue, Services, Net | Customer Concentration Risk | Comcast Corporation
Concentration Risk
Contract revenues	$ 171,200	$ 153,100	$ 330,300	$ 306,000
Concentration risk percentage	21.40%	19.60%	21.60%	19.50%
Sales Revenue, Services, Net | Customer Concentration Risk | Verizon Communications Inc.
Concentration Risk
Contract revenues	$ 147,300	$ 78,300	$ 269,500	$ 145,100
Concentration risk percentage	18.40%	10.00%	17.60%	9.30%
Sales Revenue, Services, Net | Customer Concentration Risk | CenturyLink, Inc.(1)
Concentration Risk
Contract revenues	$ 107,600	$ 165,200		$ 311,500
Concentration risk percentage	13.50%	21.20%	12.90%	19.90%
Sales Revenue, Services, Net | Customer Concentration Risk | Total other customers combined
Concentration Risk
Contract revenues	$ 208,200	$ 220,100	$ 391,600	$ 427,300
Concentration risk percentage	26.00%	28.20%	25.50%	27.30%