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		9 Months Ended
	Jan. 24, 2014	Jan. 25, 2013	Jan. 24, 2014	Jan. 25, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 39.5	$ 46.5	$ 42.0	$ 51.0
Total unrealized gains, net included in other comprehensive income (loss)	0.9	0	0.7	0.5
Total realized gains included in earnings	0	0.7	0.7	0.7
Sales	0	0	(3.0)	0
Settlements	0	(5.0)	0	(10.0)
Balance at end of period	$ 40.4	$ 42.2	$ 40.4	$ 42.2