Service Revenue (Summary of Changes in Deferred Revenue Related to Material Right Performance Obligations) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Service Revenue [Abstract]
Balance, beginning of period	$ 45.1	$ 46.0	$ 45.7	$ 46.4
Deferral of revenue	7.4	7.7	20.3	20.9
Recognition of unearned revenue	(7.4)	(7.6)	(20.9)	(21.2)
Balance, end of period	$ 45.1	$ 46.1	$ 45.1	$ 46.1