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Debt - Components of the Convertible Notes (Details) - USD ($)		6 Months Ended	12 Months Ended
	Sep. 15, 2015	Jan. 27, 2018	Jan. 26, 2019	Jul. 29, 2017	Jul. 30, 2016
Debt Instrument [Line Items]
Proceeds from sale of warrants		$ 0	$ 0	$ 0	$ 74,690,000
Payments for Hedge, Financing Activities		0	0	0	115,818,000
Equity component of 0.75% senior convertible notes due 2021, net			112,600,000
Amortization of debt discount		9,170,000	19,103,000	17,610,000	14,709,000
Estimate of Fair Value Measurement
Debt Instrument [Line Items]
Debt instrument, face amount		659,649,000	467,104,000
Less: Debt discount and debt issuance costs		(82,751,000)	(61,801,000)
Net carrying amount of Notes		576,898,000	405,303,000
0.75% Convertible Senior Notes Due 2021
Debt Instrument [Line Items]
Proceeds from sale of warrants	$ 74,700,000
Debt instrument, face amount	485,000,000.0	485,000,000	485,000,000
Debt Instrument, Unamortized Discount		(74,899,000)	(55,795,000)
Debt issuance cost		(7,852,000)	(6,006,000)
Net carrying amount of Notes		402,249,000	423,199,000
Amortization of debt discount		$ 9,200,000	$ 19,100,000	$ 17,600,000	$ 14,700,000
Convertible Note Hedge
Debt Instrument [Line Items]
Payments for Hedge, Financing Activities	$ 115,800,000