Risk Management Activities (Schedule of unallocated corporate items) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 25, 2018	Feb. 26, 2017	Feb. 25, 2018	Feb. 26, 2017
Commodity Price Risk [Abstract]
Net gain (loss) on mark-to-market valuation of certain commodity positions	$ 0.3	$ 0.0	$ (8.1)	$ (15.9)
Net loss on commodity positions reclassified from unallocated corporate items to segment operating profit	4.6	4.0	10.7	27.7
Net mark-to-market revaluation of certain grain inventories	(7.7)	4.2	0.9	8.9
Net mark-to-market valuation of certain commodity positions recognized in unallocated corporate items	$ (2.8)	$ 8.2	$ 3.5	$ 20.7