Service Revenue (Summary of Changes in Deferred Costs to Obtain and Fulfill Contracts) (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2021	Aug. 31, 2020
Capitalized Contract Cost [Line Items]
Amortization	$ 49.1	$ 47.3
Costs To Fulfill Contracts [Member]
Capitalized Contract Cost [Line Items]
Balance, beginning of period	69.3	67.3
Capitalization of costs	7.1	6.1
Amortization	(6.2)	(5.9)
Balance, end of period	70.2	67.5
Costs To Obtain Contracts [Member]
Capitalized Contract Cost [Line Items]
Balance, beginning of period	488.2	473.6
Capitalization of costs	46.6	37.2
Amortization	(42.9)	(41.4)
Balance, end of period	$ 491.9	$ 469.4