Description of Business, Basis of Presentation, and Significant Accounting Policies (Schedule of Recast of Selected Consolidated Balance Sheet) (Details) - USD ($) $ in Millions		May 31, 2019	May 31, 2018
Assets
Accounts receivable, net of allowance for doubtful accounts		$ 420.5	$ 374.6	[1]
PEO unbilled receivables, net of advance collections		406.3	117.8	[1],[2]
Prepaid expenses and other current assets		233.9	224.0	[1]
Long-term deferred costs		366.3	361.0	[1],[3]
Liabilities and stockholders' equity
Accounts payable		75.9	73.7	[1]
Deferred revenue		40.3	34.6	[1]
Deferred income taxes		223.1	154.4	[1]
Other long-term liabilities		164.7	89.1	[1]
Retained earnings		$ 1,409.5	1,262.6	[1]
As Previously Reported [Member]
Assets
Accounts receivable, net of allowance for doubtful accounts			413.6
PEO unbilled receivables, net of advance collections	[2]		117.8
Prepaid expenses and other current assets			75.8
Long-term deferred costs	[3]		18.5
Liabilities and stockholders' equity
Accounts payable			74.5
Deferred revenue			24.3
Deferred income taxes			48.8
Other long-term liabilities			84.8
Retained earnings			930.3
ASU 2014-09 [Member] | Adjustments [Member]
Assets
Accounts receivable, net of allowance for doubtful accounts			(39.0)
Prepaid expenses and other current assets			148.2
Long-term deferred costs	[3]		342.5
Liabilities and stockholders' equity
Accounts payable			(0.8)
Deferred revenue			10.3
Deferred income taxes			105.6
Other long-term liabilities			4.3
Retained earnings			$ 332.3

[1] Amounts have been adjusted to reflect the adoption of ASC Topic 606 and to provide further detail related to certain professional employer organization ("PEO") balances.

[2] Amounts were previously reported as a component of accounts receivable, net of allowance for doubtful accounts included in the Company's fiscal 2018 Form 10-K. PEO unbilled receivables, net of advance collections are separately presented on the Consolidated Balance Sheets contained in this Form 10-K.

[3] Amounts were previously reported as a component of other long-term assets on the Consolidated Balance Sheets included in the Company's fiscal 2018 Form 10-K. Long-term deferred costs are separately presented on the Consolidated Balance Sheets contained in this Form 10-K.