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Service Revenue (Summary of Disaggregates Revenue by Services) (Details) - USD ($)
$ in Millions
3 Months Ended
Aug. 31, 2018
Aug. 31, 2017
[1]
Disaggregation of Revenue [Line Items]    
Total service revenue $ 845.7 $ 779.2
Management Solutions [Member]    
Disaggregation of Revenue [Line Items]    
Total service revenue 687.7 665.9
PEO And Insurance Services [Member]    
Disaggregation of Revenue [Line Items]    
Total service revenue $ 158.0 $ 113.3
[1] Amounts have been adjusted to reflect the adoption of Accounting Standards Codification ("ASC") Topic 606, "Revenue from Contracts with Customers" ("ASC Topic 606").