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		9 Months Ended
	Jan. 25, 2013	Jan. 27, 2012	Jan. 25, 2013	Jan. 27, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 46.5	$ 56.0	$ 51.0	$ 65.1
Total unrealized gains (losses), net included in other comprehensive income (loss)	0	(0.7)	0.5	0.6
Total realized gains included in earnings	0.7	0.7	0.7	0.7
Settlements	(5.0)	(5.0)	(10.0)	(15.4)
Balance at end of period	$ 42.2	$ 51.0	$ 42.2	$ 51.0