Description of Business, Basis of Presentation, and Significant Accounting Policies (Schedule of Recast of Selected Unaudited Consolidated Statements of Income and Comprehensive Income) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue	$ 845.7	$ 779.2	[1]
Operating expenses	265.5	232.0	[1]
Selling, general and administrative expenses	277.0	243.6	[1]
Total expenses	542.5	475.6	[1]
Operating income	320.3	317.3	[1]
Income taxes	79.0	109.0	[1]
Net income	$ 243.6	$ 210.4	[1],[2]
Basic earnings per share	$ 0.68	$ 0.59	[1],[2]
Diluted earnings per share	$ 0.67	$ 0.58	[1],[2]
As Previously Reported [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue		$ 803.1
Operating expenses		232.1
Selling, general and administrative expenses		239.7
Total expenses		471.8
Operating income		345.0
Income taxes		119.3
Net income		$ 227.8
Basic earnings per share		$ 0.63
Diluted earnings per share		$ 0.63
Adjustments [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue		$ (23.9)
Operating expenses		(0.1)
Selling, general and administrative expenses		3.9
Total expenses		3.8
Operating income		(27.7)
Income taxes		(10.3)
Net income		$ (17.4)
Basic earnings per share		$ (0.04)
Diluted earnings per share		$ (0.05)

[1] Amounts have been adjusted to reflect the adoption of Accounting Standards Codification ("ASC") Topic 606, "Revenue from Contracts with Customers" ("ASC Topic 606").

[2] Amounts have been adjusted to reflect the adoption of ASC Topic 606.