Financial Instruments and Fair Value - 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. 26, 2012	Oct. 28, 2011	Oct. 26, 2012	Oct. 28, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 46.3	$ 56.3	$ 51.0	$ 65.1
Total unrealized gains, net included in other comprehensive income (loss)	0.2	0.4	0.5	1.3
Settlements	0	(0.7)	(5.0)	(10.4)
Balance at end of period	$ 46.5	$ 56.0	$ 46.5	$ 56.0