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Concentration of Credit Risk - Revenue Concentration Risk (Details) - Sales Revenue, Services, Net - Customer Concentration Risk | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 27, 2018 | Jul. 29, 2017 | Jul. 30, 2016 | Jul. 25, 2015
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage | 10.00% |  |  |
Comcast Corporation |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage | 21.60% | 17.70% | 13.60% | 12.90%
AT&T Inc. |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage | 20.60% | 26.30% | 24.40% | 20.80%
CenturyLink, Inc.(1) |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage | 17.50% | 18.20% | 14.70% | 14.50%
Verizon Communications Inc |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Concentration risk percentage | 12.00% | 9.20% | 11.20% | 7.70%

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