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
Nov. 30, 2017
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 $ (12.9) $ (5.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (9.8)  
Total, Gross unrealized losses (22.7) (5.8)
Securities in an unrealized loss position for less than twelve months, Fair value 1,522.5 606.4
Securities in an unrealized loss position for more than twelve months, Fair value 348.3 1.0
Total, Fair value 1,870.8 607.4
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 (1.0) (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.7)  
Total, Gross unrealized losses (1.7) (0.5)
Securities in an unrealized loss position for less than twelve months, Fair value 117.1 43.6
Securities in an unrealized loss position for more than twelve months, Fair value 35.3  
Total, Fair value 152.4 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 (5.5) (0.9)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (2.1)  
Total, Gross unrealized losses (7.6) (0.9)
Securities in an unrealized loss position for less than twelve months, Fair value 723.2 188.8
Securities in an unrealized loss position for more than twelve months, Fair value 76.3  
Total, Fair value 799.5 188.8
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 13.4 9.2
Securities in an unrealized loss position for more than twelve months, Fair value 5.8  
Total, Fair value 19.2 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 (4.3) (0.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.4)  
Total, Gross unrealized losses (5.7) (0.8)
Securities in an unrealized loss position for less than twelve months, Fair value 478.0 154.8
Securities in an unrealized loss position for more than twelve months, Fair value 52.4 1.0
Total, Fair value 530.4 155.8
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 (2.0) (3.6)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (5.6)  
Total, Gross unrealized losses (7.6) (3.6)
Securities in an unrealized loss position for less than twelve months, Fair value 190.8 210.0
Securities in an unrealized loss position for more than twelve months, Fair value 178.5  
Total, Fair value $ 369.3 $ 210.0