Service Revenue (Summary of Changes in Deferred Costs to Obtain and Fulfill Contracts) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2020	Nov. 30, 2019	Nov. 30, 2020	Nov. 30, 2019
Capitalized Contract Cost [Line Items]
Amortization			$ 95.1	$ 92.4
Costs To Obtain A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	$ 469.4	$ 460.0	473.6	464.3
Capitalization of costs	40.4	37.4	77.6	73.3
Amortization	(41.7)	(40.5)	(83.1)	(80.7)
Ending balance	468.1	456.9	468.1	456.9
Costs To Fulfill A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	67.5	66.3	67.3	66.1
Capitalization of costs	6.0	6.0	12.1	12.0
Amortization	(6.1)	(5.9)	(12.0)	(11.7)
Ending balance	$ 67.4	$ 66.4	$ 67.4	$ 66.4