Annual report pursuant to Section 13 and 15(d)

Description of Business, Basis of Presentation, and Significant Accounting Policies (Schedule of Recast of Selected Consolidated Balance Sheet) (Details)

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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.