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			9 Months Ended
	Feb. 28, 2019	Feb. 28, 2018		Feb. 28, 2019		Feb. 28, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue	$ 1,047.4	$ 918.0	[1]	$ 2,733.7		$ 2,486.2	[1]
Operating expenses	325.3	272.2	[1]	855.7		752.8	[1]
Selling, general and administrative expenses	315.8	294.9	[1]	879.6		789.8	[1]
Total expenses	641.1	567.1	[1]	1,735.3		1,542.6	[1]
Operating income	429.3	369.0	[1]	1,056.8		989.4	[1]
Income taxes	101.0	4.1	[1]	253.5		219.1	[1]
Net income	$ 324.6	$ 367.2	[1],[2]	$ 804.0	[3]	$ 776.4	[1],[2],[3]
Basic earnings per share	$ 0.90	$ 1.02	[1],[2]	$ 2.24		$ 2.16	[1],[2]
Diluted earnings per share	$ 0.90	$ 1.01	[1],[2]	$ 2.22		$ 2.15	[1],[2]
As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income						$ 705.2
As Previously Reported [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue		$ 848.4				2,464.0
Operating expenses		270.7				751.5
Selling, general and administrative expenses		303.3				788.6
Total expenses		574.0				1,540.1
Operating income		292.5				969.7
Income taxes		34.4				270.6
Net income		$ 260.4				$ 705.2
Basic earnings per share		$ 0.72				$ 1.96
Diluted earnings per share		$ 0.72				$ 1.95
Adjustments [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service revenue		$ 69.6				$ 22.2
Operating expenses		1.5				1.3
Selling, general and administrative expenses		(8.4)				1.2
Total expenses		(6.9)				2.5
Operating income		76.5				19.7
Income taxes		(30.3)				(51.5)
Net income		$ 106.8				$ 71.2
Basic earnings per share		$ 0.30				$ 0.20
Diluted earnings per share		$ 0.29				$ 0.20

[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.

[3] Amounts have been adjusted to reflect the adoption of Accounting Standards Update ("ASU") No. 2016-18, "Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force)."