Annual report pursuant to Section 13 and 15(d)

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

v3.10.0.1
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
May 31, 2018
May 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses $ (27.5) $ (5.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (13.2)  
Total, Gross unrealized losses (40.7) (5.8)
Securities in an unrealized loss position for less than twelve months, Fair value 1,907.0 606.4
Securities in an unrealized loss position for more than twelve months, Fair value 331.3 1.0
Total, Fair value 2,238.3 607.4
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 (6.0) (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.3)  
Total, Gross unrealized losses (7.3) (0.5)
Securities in an unrealized loss position for less than twelve months, Fair value 271.5 43.6
Securities in an unrealized loss position for more than twelve months, Fair value 34.6  
Total, Fair value 306.1 43.6
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 (8.7) (0.9)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (2.5)  
Total, Gross unrealized losses (11.2) (0.9)
Securities in an unrealized loss position for less than twelve months, Fair value 856.4 188.8
Securities in an unrealized loss position for more than twelve months, Fair value 68.9  
Total, Fair value 925.3 188.8
Pre-Refunded 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)  
Total, Gross unrealized losses (0.1)  
Securities in an unrealized loss position for less than twelve months, Fair value 18.5 9.2
Securities in an unrealized loss position for more than twelve months, Fair value 0.5  
Total, Fair value 19.0 9.2
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 (6.3) (0.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.7)  
Total, Gross unrealized losses (8.0) (0.8)
Securities in an unrealized loss position for less than twelve months, Fair value 540.9 154.8
Securities in an unrealized loss position for more than twelve months, Fair value 50.9 1.0
Total, Fair value 591.8 155.8
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 (6.4) (3.6)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (7.7)  
Total, Gross unrealized losses (14.1) (3.6)
Securities in an unrealized loss position for less than twelve months, Fair value 219.7 210.0
Securities in an unrealized loss position for more than twelve months, Fair value 176.4  
Total, Fair value $ 396.1 $ 210.0