Financing Arrangements - Amount of Interest Expense Recognized Related to Convertible Notes (Detail) (Convertible Notes Payable, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 25, 2013	Jan. 27, 2012	Jan. 25, 2013	Jan. 27, 2012
Convertible Notes Payable
Debt Instrument [Line Items]
Contractual coupon interest expense	$ 5.5	$ 5.5	$ 16.5	$ 16.5
Amortization of debt discount	13.9	13.0	41.0	38.4
Amortization of debt issuance costs	1.2	1.1	3.6	3.3
Less capitalized interest	0	(0.9)	(1.1)	(4.3)
Total interest expense related to Convertible Notes	$ 20.6	$ 18.7	$ 60.0	$ 53.9