Annual report pursuant to Section 13 and 15(d)

Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details)

v3.5.0.2
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
May 31, 2016
May 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses $ (0.5) $ (7.0)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.2) (0.5)
Total, Gross unrealized losses (0.7) (7.5)
Securities in an unrealized loss position for less than twelve months, Fair value 141.1 896.7
Securities in an unrealized loss position for more than twelve months, Fair value 14.5 26.3
Total, Fair value 155.6 923.0
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses (0.1)  
Total, Gross unrealized losses (0.1)  
Securities in an unrealized loss position for less than twelve months, Fair value 14.7  
Total, Fair value 14.7  
General Obligation Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses (0.1) (3.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.5)
Total, Gross unrealized losses (0.1) (4.3)
Securities in an unrealized loss position for less than twelve months, Fair value 48.9 535.1
Securities in an unrealized loss position for more than twelve months, Fair value 2.8 26.3
Total, Fair value 51.7 561.4
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Fair value 5.7  
Total, Fair value 5.7  
Revenue Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses   (3.2)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.2)  
Total, Gross unrealized losses (0.2) (3.2)
Securities in an unrealized loss position for less than twelve months, Fair value 20.7 361.6
Securities in an unrealized loss position for more than twelve months, Fair value 11.7  
Total, Fair value 32.4 $ 361.6
U.S. Government Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses (0.3)  
Total, Gross unrealized losses (0.3)  
Securities in an unrealized loss position for less than twelve months, Fair value 51.1  
Total, Fair value $ 51.1