Financial Instruments, Risk Management Activities, and Fair Values (Schedule of maturities of available for sale securities) (Details) - USD ($) $ in Millions	May 28, 2017	May 29, 2016
Schedule of Available-for-Sale Securities [Line Items]
Cost	$ 267.2	$ 167.5
Fair Value	275.4	174.2
Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Cost	265.4	165.7
Fair Value	265.5	165.8
Debt Securities [Member] | Available-for-Sale Securities Debt Maturities Within One Year [Member]
Schedule of Available-for-Sale Securities [Line Items]
Cost	265.4
Fair Value	265.5
Equity Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Cost	1.8	1.8
Fair Value	$ 9.9	$ 8.4