Long-term debt (Details) (USD $)	3 Months Ended
	Apr. 26, 2015		Apr. 27, 2014	Jan. 25, 2015
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
Principal amount of Notes	1,000
Debt Instrument, Convertible, Conversion Price	$ 20.16
Debt Instrument, Convertible, Initial Liability Amount	1,350,000,000
Debt Instrument, Effective Interest Rate	3.15%
Debt Instrument, Convertible, Carrying Amount of Equity Component	126,000,000			126,000,000
Purchaser's Discount of Convertible Notes	23,000,000
Initial unamortized debt discount at issuance	149,000,000
Debt Instrument, Unamortized Discount	(109,000,000)	[1]		(116,000,000)
Long-term debt	1,391,000,000			1,384,000,000
Debt Instrument, Convertible, Remaining Discount Amortization Period	3 years 7 months 6 days
Coupon Interest Expense	4,000,000		4,000,000
Amortization of debt discount	7,000,000		7,000,000
Debt Instrument, Convertible, Interest Expense	$ 11,000,000		$ 11,000,000

[1] As of April 26, 2015, the unamortized debt discount will be amortized over a remaining period of 3.6 years.