Long-term debt (Details)	3 Months Ended			9 Months Ended
	Oct. 25, 2015 USD ($) $ / shares		Oct. 26, 2014 USD ($)	Oct. 25, 2015 USD ($) $ / shares		Oct. 26, 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				49.5958
Principal amount of Notes	$ 1,000			$ 1,000
Debt Instrument, Convertible, Conversion Price | $ / shares	$ 20.1630			$ 20.1630
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	(94,000,000)	[1]		(94,000,000)	[1]		(116,000,000)
Long-term debt	1,406,000,000			$ 1,406,000,000			$ 1,384,000,000
Debt Instrument, Convertible, Remaining Discount Amortization Period				3 years 1 month 6 days
Coupon Interest Expense	4,000,000		$ 4,000,000	$ 11,000,000		$ 11,000,000
Amortization of debt discount	7,000,000		7,000,000	22,000,000		21,000,000
Debt Instrument, Convertible, Interest Expense	$ 11,000,000		$ 11,000,000	$ 33,000,000		$ 32,000,000

[1] As of October 25, 2015, the unamortized debt discount will be amortized over a remaining period of 3.1 years.