Service Revenue (Summary of Changes in Deferred Revenue Related to Material Right Performance Obligations) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018
Service Revenue [Abstract]
Balance, beginning of period	$ 44.5	$ 45.7	$ 45.7	$ 46.4
Deferral of revenue	7.4	7.1	12.9	13.2
Recognition of unearned revenue	(6.8)	(6.8)	(13.5)	(13.6)
Balance, end of period	$ 45.1	$ 46.0	$ 45.1	$ 46.0