Long-term debt (Details) (USD $)	3 Months Ended			6 Months Ended			12 Months Ended
	Jul. 27, 2014		Jul. 28, 2013	Jul. 27, 2014		Jul. 28, 2013	Jan. 26, 2014
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,500,000,000			$ 1,500,000,000			$ 1,500,000,000
Debt Instrument, Interest Rate, Stated Percentage	1.00%			1.00%
Debt Instrument, Convertible, Conversion Ratio				49.60
Debt Instrument, Convertible, Conversion Price	$ 20.16			$ 20.16
Debt Instrument, Convertible, Terms of Conversion Feature				(1) during any fiscal quarter commencing after the fiscal quarter ended on April 27, 2014 (and only during such fiscal quarter), if the last reported sale price of the common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding fiscal quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any five consecutive trading day period (the measurement period) in which the trading price per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of our common stock and the conversion rate on each such trading day; or (3) upon the occurrence of specified corporate events. On or after August 1, 2018 to the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert all or any portion of their notes regardless of the foregoing conditions. Upon conversion, we will pay cash up to the aggregate principal amount of the notes to be converted and pay or deliver, as the case may be, cash, shares of our common stock or a combination of cash and shares of our common stock, at our election, in respect of the remainder, if any, of our conversion obligation in excess of the aggregate principal amount of the notes being converted.
Debt Instrument, Convertible, Initial Liability Amount							1,351,800,000
Debt Instrument, Effective Interest Rate	3.15%			3.15%
Debt Instrument, Convertible, Carrying Amount of Equity Component	125,725,000			125,725,000			125,725,000
Purchaser's Discount of Convertible Notes				22,500,000
Initial unamortized debt discount at issuance				148,200,000
Debt Instrument, Unamortized Discount	(129,751,000)	[1]		(129,751,000)	[1]		(143,625,000)
Long-term Debt	1,370,249,000			1,370,249,000			1,356,375,000
Debt Instrument, Convertible, Remaining Discount Amortization Period				4 years 4 months 24 days
Coupon Interest Expense	3,750,000		0	7,500,000		0
Amortization of Debt Discount	6,973,000		0	13,874,000		0
Amortization of Financing Costs	49,000		0	97,000		0
Debt Instrument, Convertible, Interest Expense	$ 10,772,000		$ 0	$ 21,471,000		$ 0

[1] As of July 27, 2014, the unamortized debt discount will be amortized over a remaining period of 4.4 years.