Long-term debt (Details)	3 Months Ended			6 Months Ended
	Jul. 26, 2015 USD ($) $ / shares		Jul. 27, 2014 USD ($)	Jul. 26, 2015 USD ($) $ / shares		Jul. 27, 2014 USD ($)	Jan. 25, 2015 USD ($)
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
Principal amount of Notes	$ 1,000			$ 1,000
Debt Instrument, Convertible, Conversion Price | $ / shares	$ 20.16			$ 20.16
Debt Instrument, Convertible, Initial Liability Amount	$ 1,350,000,000
Debt Instrument, Effective Interest Rate	3.15%			3.15%
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 126,000,000			$ 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	(101,000,000)	[1]		(101,000,000)	[1]		(116,000,000)
Long-term debt	1,399,000,000			$ 1,399,000,000			$ 1,384,000,000
Debt Instrument, Convertible, Remaining Discount Amortization Period				3 years 4 months 24 days
Coupon Interest Expense	4,000,000		$ 4,000,000	$ 8,000,000		$ 8,000,000
Amortization of debt discount	7,000,000		7,000,000	14,000,000		14,000,000
Debt Instrument, Convertible, Interest Expense	$ 11,000,000		$ 11,000,000	$ 22,000,000		$ 22,000,000

[1] As of July 26, 2015, the unamortized debt discount will be amortized over a remaining period of 3.4 years.