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	12 Months Ended
	Apr. 24, 2015	Apr. 25, 2014	Apr. 26, 2013
Fair Value Disclosures [Abstract]
Balance at beginning of year	$ 36.0	$ 42.0	$ 51.0
Total unrealized gains, net included in other comprehensive income (loss)	0.9	1.3	0.5
Total realized gains included in earnings	0	0.7	1.1
Sales	(10.0)	(8.0)	0
Settlements	(26.9)	0	(10.6)
Balance at end of year	$ 0	$ 36.0	$ 42.0