Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
Feb. 28, 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 $ (26.2) $ (5.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (13.7)  
Total, Gross unrealized losses (39.9) (5.8)
Securities in an unrealized loss position for less than twelve months, Fair value 1,649.8 606.4
Securities in an unrealized loss position for more than twelve months, Fair value 334.3 1.0
Total, Fair value 1,984.1 607.4
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 (5.2) (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.3)  
Total, Gross unrealized losses (6.5) (0.5)
Securities in an unrealized loss position for less than twelve months, Fair value 252.1 43.6
Securities in an unrealized loss position for more than twelve months, Fair value 34.7  
Total, Fair value 286.8 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 (8.0) (0.9)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (2.6)  
Total, Gross unrealized losses (10.6) (0.9)
Securities in an unrealized loss position for less than twelve months, Fair value 735.5 188.8
Securities in an unrealized loss position for more than twelve months, Fair value 71.4  
Total, Fair value 806.9 188.8
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 7.2 9.2
Securities in an unrealized loss position for more than twelve months, Fair value 0.5  
Total, Fair value 7.7 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 (6.1) (0.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.8)  
Total, Gross unrealized losses (7.9) (0.8)
Securities in an unrealized loss position for less than twelve months, Fair value 435.7 154.8
Securities in an unrealized loss position for more than twelve months, Fair value 51.6 1.0
Total, Fair value 487.3 155.8
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 (6.8) (3.6)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (8.0)  
Total, Gross unrealized losses (14.8) (3.6)
Securities in an unrealized loss position for less than twelve months, Fair value 219.3 210.0
Securities in an unrealized loss position for more than twelve months, Fair value 176.1  
Total, Fair value $ 395.4 $ 210.0