Annual report pursuant to Section 13 and 15(d)

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

v3.2.0.727
Description of Business, Basis of Presentation, and Significant Accounting Policies (Details) - USD ($)
$ in Millions
 12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Accounting Policies [Abstract]      
Disclosure on geographic areas, Description of revenue from external customers revenue within Germany, which represented less than one percent of the Company's total revenue revenue within Germany, which represented less than one percent of the Company's total revenue revenue within Germany, which represented less than one percent of the Company's total revenue
Disclosure on geographic areas, Long-lived assets Long-lived assets in Germany are insignificant in relation to total long-lived assets of the Company Long-lived assets in Germany are insignificant in relation to total long-lived assets of the Company  
Minimum period in which payroll taxes are typically remitted after receipt of collections from client 1 day    
Maximum period in which payroll taxes are typically remitted after receipt of collections from client 30 days    
Maximum Days in Which Payroll Taxes Will Be Remitted After Receipt Of Collections From Client 90 days    
Allowance for doubtful accounts $ 1.4 $ 1.5  
Direct costs billed and incurred 4,200.0 3,400.0 $ 3,000.0
Maximum individual workers' compensation claims liability 1.0 1.0  
Maximum Individual Claims Liability Health Insurance 0.3 0.3  
Workers' compensation insurance costs in current liabilities 11.3 8.5  
Workers' compensation insurance costs in long-term liabilities 15.3 16.0  
PEO health insurance costs in current liabilities 8.1 4.6  
Reserve for uncertain tax positions 29.1 29.8  
Property, Plant and Equipment [Line Items]      
Goodwill $ 561.5 $ 540.3  
Software Development [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 15 years    
Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Finite-Lived Intangible Asset, Useful Life 3 years    
Minimum [Member] | Building and Building Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 10 years    
Minimum [Member] | Data Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 3 years    
Minimum [Member] | Furniture Fixtures And Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 3 years    
Minimum [Member] | Software and Software Development Costs [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 3 years    
Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Finite-Lived Intangible Asset, Useful Life 12 years    
Maximum [Member] | Building and Building Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 35 years    
Maximum [Member] | Data Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 4 years    
Maximum [Member] | Furniture Fixtures And Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 7 years    
Maximum [Member] | Leasehold Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 10 years    
Maximum [Member] | Software and Software Development Costs [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment, Useful Life 5 years