Long-term debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Jan. 25, 2015	Jan. 26, 2014	Jan. 27, 2013
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,500,000,000	$ 1,500,000,000
Debt Instrument, Interest Rate, Stated Percentage	1.00%
Debt Instrument, Convertible, Conversion Ratio	49.60
Debt Instrument, Convertible, Conversion Price	$ 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, Convertible, Effective Interest Rate	3.15%
Discount on convertible notes	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	(115,658,000)	(143,625,000)
Long-term Debt	1,384,342,000	1,356,375,000
Debt Instrument, Convertible, Remaining Discount Amortization Period	3 years 10 months 24 days
Coupon Interest Expense	15,000,000	2,500,000
Amortization of debt discount	27,967,000	4,600,000	0
Amortization of Financing Costs	195,000	34,000
Debt Instrument, Convertible, Interest Expense	43,162,000	7,134,000
Proceeds from issuance of convertible notes, net	0	1,477,500,000	0
Note Hedges Strike Price	$ 20.16
Purchase of convertible note hedges	0	167,100,000	0
Warrant Strike Price		$ 27.14
Number of Shares Covered by Warrants	74.4
Proceeds from the sale of common stock warrants	0	59,100,000	0
Net Cost of Hedges and Warrants Transactions		$ 108,000,000