Service Revenue (Summary of Changes in Deferred Revenue Related to Material Rights Performance Obligations) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 28, 2021	Feb. 29, 2020	Feb. 28, 2021	Feb. 29, 2020
Service Revenue [Abstract]
Balance, beginning of period	$ 39.6	$ 45.1	$ 42.6	$ 45.7
Deferral of revenue	6.9	7.4	16.6	20.3
Recognition of unearned revenue	(6.7)	(7.4)	(19.4)	(20.9)
Balance, end of period	$ 39.8	$ 45.1	$ 39.8	$ 45.1