Service Revenue (Summary of Changes in Deferred Costs to Obtain and Fulfill Contracts) (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2020	Aug. 31, 2019
Capitalized Contract Cost [Line Items]
Amortization	$ 47.3	$ 46.0
Costs To Obtain A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	473.6	464.3
Capitalization of costs	37.2	35.9
Amortization	(41.4)	(40.2)
Ending balance	469.4	460.0
Costs To Fulfill A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	67.3	66.1
Capitalization of costs	6.1	6.0
Amortization	(5.9)	(5.8)
Ending balance	$ 67.5	$ 66.3