Financial Instruments and Fair Value Measurements - Schedule of Gross Unrealized Losses and Estimated Fair Values of Investments (Detail) (USD $) In Millions, unless otherwise specified	Apr. 25, 2014	Apr. 26, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months estimated fair value	$ 296.1	$ 698.1
Less than 12 months, unrealized loss	(0.6)	(0.5)
12 months or greater estimated fair value	36.0	77.6
12 months or greater, unrealized loss	(0.9)	(2.8)
Estimated fair value, total	332.1	775.7
Unrealized loss, total	(1.5)	(3.3)
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months estimated fair value	233.1	698.1
Less than 12 months, unrealized loss	(0.5)	(0.5)
12 months or greater estimated fair value	0	38.4
12 months or greater, unrealized loss	0	(0.1)
Estimated fair value, total	233.1	736.5
Unrealized loss, total	(0.5)	(0.6)
Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months estimated fair value	0	0
Less than 12 months, unrealized loss	0	0
12 months or greater estimated fair value	36.0	39.2
12 months or greater, unrealized loss	(0.9)	(2.7)
Estimated fair value, total	36.0	39.2
Unrealized loss, total	(0.9)	(2.7)
US Treasury and Government Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months estimated fair value	63.0
Less than 12 months, unrealized loss	(0.1)
12 months or greater estimated fair value	0
12 months or greater, unrealized loss	0
Estimated fair value, total	63.0
Unrealized loss, total	$ (0.1)