Financial Instruments and Fair Value Measurements - Reconciliation of Beginning and Ending Balance of Level Three Auction Rate Securities Measured at Fair Value On Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 24, 2014	Oct. 25, 2013	Oct. 24, 2014	Oct. 25, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 26.4	$ 42.2	$ 36.0	$ 42.0
Total unrealized gains (losses), net included in other comprehensive income (loss)	0.2	(0.4)	0.6	(0.2)
Total realized gains included in earnings	0	0.7	0	0.7
Sales	0	(3.0)	(10.0)	(3.0)
Settlements	(3.6)	0	(3.6)	0
Balance at end of period	$ 23.0	$ 39.5	$ 23.0	$ 39.5