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			6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017		Nov. 30, 2018	Nov. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue	$ 840.6	$ 789.0	[1]	$ 1,686.3	$ 1,568.2	[1]
Operating expenses	264.9	248.6	[1]	530.4	480.6	[1]
Selling, general and administrative expenses	286.8	251.3	[1]	563.8	494.9	[1]
Total expenses	551.7	499.9	[1]	1,094.2	975.5	[1]
Operating income	307.2	303.1	[1]	627.5	620.4	[1]
Income taxes	73.5	106.0	[1]	152.5	215.0	[1]
Net income	$ 235.8	$ 198.8	[1],[2]	$ 479.4	$ 409.2	[1],[2]
Basic earnings per share	$ 0.66	$ 0.55	[1],[2]	$ 1.34	$ 1.14	[1],[2]
Diluted earnings per share	$ 0.65	$ 0.55	[1],[2]	$ 1.33	$ 1.13	[1],[2]
As Previously Reported [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue		$ 812.5			$ 1,615.6
Operating expenses		248.7			480.8
Selling, general and administrative expenses		245.6			485.3
Total expenses		494.3			966.1
Operating income		332.2			677.2
Income taxes		116.9			236.2
Net income		$ 217.0			$ 444.8
Basic earnings per share		$ 0.60			$ 1.24
Diluted earnings per share		$ 0.60			$ 1.23
Adjustments [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue		$ (23.5)			$ (47.4)
Operating expenses		(0.1)			(0.2)
Selling, general and administrative expenses		5.7			9.6
Total expenses		5.6			9.4
Operating income		(29.1)			(56.8)
Income taxes		(10.9)			(21.2)
Net income		$ (18.2)			$ (35.6)
Basic earnings per share		$ (0.05)			$ (0.10)
Diluted earnings per share		$ (0.05)			$ (0.10)

[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.