Annual report pursuant to Section 13 and 15(d)

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

v3.5.0.2
Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
May 31, 2016
USD ($)
segment
$ / shares
May 31, 2015
USD ($)
May 31, 2014
USD ($)
Jul. 07, 2016
$ / shares
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      
Declared date Jul. 07, 2016      
Quarterly dividend per share | $ / shares $ 0.42      
Payable date Aug. 25, 2016      
Record date Aug. 01, 2016      
Allowance for doubtful accounts $ 4.2 $ 1.4    
Accounts receivable, net of allowance for doubtful accounts 408.6 176.6    
Goodwill 657.1 561.5    
Goodwill impairment loss 0.0 0.0 $ 0.0  
Direct pass-through costs 5,100.0 4,200.0 $ 3,400.0  
Maximum individual workers' compensation claims liability 1.3 1.0    
Workers' compensation insurance costs in current liabilities 13.8 11.3    
Workers' compensation insurance costs in long-term liabilities 18.4 15.3    
Maximum individual health insurance claims liability 0.3 0.3    
PEO accrued health insurance reserves in current liabilities 9.5 8.1    
Reserve for uncertain tax positions $ 54.2 29.1    
Software Development [Member]        
Accounting Policies [Line Items]        
Estimated useful lives 15 years      
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 45 days      
Amortization period 3 years      
Minimum [Member] | Software [Member]        
Accounting Policies [Line Items]        
Estimated useful lives 3 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 60 days      
Amortization period 12 years      
Maximum [Member] | Software [Member]        
Accounting Policies [Line Items]        
Estimated useful lives 5 years      
Trade Receivables [Member]        
Accounting Policies [Line Items]        
Accounts receivable, net of allowance for doubtful accounts $ 221.6 $ 176.6    
Purchased Receivables [Member]        
Accounting Policies [Line Items]        
Accounts receivable, net of allowance for doubtful accounts $ 187.0      
Germany [Member] | Total Revenue [Member] | Geographic Concentration Risk [Member]        
Accounting Policies [Line Items]        
Percentage of total revenue less than one percent 1.00% 1.00% 1.00%  
Subsequent Event [Member]        
Accounting Policies [Line Items]        
Quarterly dividend per share increase | $ / shares       $ 0.04
Quarterly dividend percentage increase       10.00%
Quarterly dividend per share | $ / shares       $ 0.46