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
	Jul. 26, 2013	Jul. 27, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 42.0	$ 51.0
Total unrealized gains, net included in other comprehensive income (loss)	0.2	0.3
Settlements	0	(5.0)
Balance at end of period	$ 42.2	$ 46.3