Service Revenue (Summary of Changes in Deferred Costs to Obtain and Fulfill Contracts) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Capitalized Contract Cost [Line Items]
Amortization			$ 139.1	$ 134.8
Costs To Obtain A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	$ 456.9	$ 447.3	464.3	455.0
Capitalization of costs	49.4	47.9	122.7	118.2
Amortization	(40.7)	(39.6)	(121.4)	(117.6)
Ending balance	465.6	455.6	465.6	455.6
Costs To Fulfill A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	66.4	65.8	66.1	65.4
Capitalization of costs	6.7	5.9	18.7	17.8
Amortization	(6.0)	(5.7)	(17.7)	(17.2)
Ending balance	$ 67.1	$ 66.0	$ 67.1	$ 66.0