Financial Instruments, Risk Management Activities, and Fair Values (Schedule of maturities of available for sale securities) (Details) (USD $) In Millions, unless otherwise specified	Nov. 23, 2014	May 25, 2014
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 249.4	$ 320.4
Market Value	255.4	326.0
Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	247.6	318.6
Market Value	247.8	318.8
Debt Securities [Member] | Maturities under one year [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	247.0
Market Value	247.2
Debt Securities [Member] | Maturities, from one to three years [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	0.6
Market Value	0.6
Debt Securities [Member] | Maturities, four to seven years [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	0
Market Value	0
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost	1.8	1.8
Market Value	$ 7.6	$ 7.2