Service Revenue (Summary of Changes in Deferred Revenue Related to Material Rights Performance Obligations) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2020	Nov. 30, 2019	Nov. 30, 2020	Nov. 30, 2019
Service Revenue [Abstract]
Balance, beginning of period	$ 40.2	$ 44.5	$ 42.6	$ 45.7
Deferral of revenue	5.7	7.4	9.7	12.9
Recognition of unearned revenue	(6.3)	(6.8)	(12.7)	(13.5)
Balance, end of period	$ 39.6	$ 45.1	$ 39.6	$ 45.1