Annual report pursuant to Section 13 and 15(d)

Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details)

v3.10.0.1
Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Jul. 12, 2018
$ / shares
May 31, 2018
USD ($)
segment
May 31, 2017
USD ($)
May 31, 2016
USD ($)
Accounting Policies [Line Items]        
Number of reportable segments | segment   1    
Maximum period in which payroll taxes will be remitted after receipt of collections from clients   90 days    
Goodwill   $ 814.0 $ 657.1  
Goodwill impairment loss   0.0 0.0 $ 0.0
Direct pass-through costs   7,900.0 5,900.0 $ 5,100.0
Maximum individual workers' compensation claims liability   1.3 1.3  
Workers' compensation insurance costs in current liabilities   19.4 18.0  
Workers' compensation insurance costs in long-term liabilities   31.2 22.3  
Maximum individual health insurance claims liability   0.3 0.3  
PEO accrued health insurance reserves in current liabilities   25.4 15.7  
Reserve for uncertain tax positions   $ 14.7 $ 39.2  
Subsequent Event [Member]        
Accounting Policies [Line Items]        
Declared date Jul. 12, 2018      
Quarterly dividend per share | $ / shares $ 0.56      
Payable date Aug. 23, 2018      
Record date Aug. 01, 2018      
Minimum [Member]        
Accounting Policies [Line Items]        
Period in which payroll taxes are typically remitted after receipt of collections from clients   1 day    
Payroll funding fees average collection period   35 days    
Minimum [Member] | Finite Lived Assets [Member]        
Accounting Policies [Line Items]        
Amortization period   3 years    
Minimum [Member] | Software [Member]        
Accounting Policies [Line Items]        
Estimated useful lives   3 years    
Minimum [Member] | Software Development [Member]        
Accounting Policies [Line Items]        
Estimated useful lives   12 years    
Maximum [Member]        
Accounting Policies [Line Items]        
Period in which payroll taxes are typically remitted after receipt of collections from clients   30 days    
Payroll funding fees average collection period   45 days    
Short-term investments maturity at acquisition   90 days    
Maximum [Member] | Finite Lived Assets [Member]        
Accounting Policies [Line Items]        
Amortization period   12 years    
Maximum [Member] | Software [Member]        
Accounting Policies [Line Items]        
Estimated useful lives   5 years    
HR Outsourcing Holdings, Inc. [Member]        
Accounting Policies [Line Items]        
Maximum individual workers' compensation claims liability   $ 0.5    
Maximum individual health insurance claims liability   $ 0.3    
Europe [Member] | Revenue [Member] | Geographic Concentration Risk [Member] | Maximum [Member]        
Accounting Policies [Line Items]        
Total percentage   1.00% 1.00% 1.00%
Europe [Member] | Long Lived Assets [Member] | Geographic Concentration Risk [Member]        
Accounting Policies [Line Items]        
Total percentage   13.00%