Description of Business, Basis of Presentation, and Significant Accounting Policies (Schedule of Recast of Selected Unaudited Consolidated Statement of Cash Flow) (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017
Cash flows from operating activities
Net income	$ 243.6	$ 210.4	[1],[2]
Provision for deferred income taxes	2.6	48.9	[1]
Accounts receivable	(43.0)	8.8	[1]
Deferred costs	6.2	4.2	[1]
Accounts payable and other current liabilities	(15.2)	(19.8)	[1]
Net change in other long-term assets and liabilities	$ (0.4)	0.5	[1]
ASU 2014-09 [Member] | As Previously Reported [Member]
Cash flows from operating activities
Net income		227.8
Provision for deferred income taxes		59.2
Accounts receivable		(15.4)
Accounts payable and other current liabilities		(19.2)
Net change in other long-term assets and liabilities		0.6
ASU 2014-09 [Member] | Adjustments [Member]
Cash flows from operating activities
Net income		(17.4)
Provision for deferred income taxes		(10.3)
Accounts receivable		24.2
Deferred costs		4.2
Accounts payable and other current liabilities		(0.6)
Net change in other long-term assets and liabilities		$ (0.1)

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

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