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
Aug. 31, 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 $ (2.4) $ (5.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.2)  
Total, Gross unrealized losses (3.6) (5.8)
Securities in an unrealized loss position for less than twelve months, Fair value 336.3 606.4
Securities in an unrealized loss position for more than twelve months, Fair value 70.1 1.0
Total, Fair value 406.4 607.4
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 (0.2) (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.1)  
Total, Gross unrealized losses (0.3) (0.5)
Securities in an unrealized loss position for less than twelve months, Fair value 34.1 43.6
Securities in an unrealized loss position for more than twelve months, Fair value 6.6  
Total, Fair value 40.7 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 (0.3) (0.9)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.1)  
Total, Gross unrealized losses (0.4) (0.9)
Securities in an unrealized loss position for less than twelve months, Fair value 78.2 188.8
Securities in an unrealized loss position for more than twelve months, Fair value 12.1  
Total, Fair value 90.3 188.8
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Fair value 9.1 9.2
Total, Fair value 9.1 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 (0.1) (0.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.1)  
Total, Gross unrealized losses (0.2) (0.8)
Securities in an unrealized loss position for less than twelve months, Fair value 70.6 154.8
Securities in an unrealized loss position for more than twelve months, Fair value 12.3 1.0
Total, Fair value 82.9 155.8
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 (1.8) (3.6)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (0.9)  
Total, Gross unrealized losses (2.7) (3.6)
Securities in an unrealized loss position for less than twelve months, Fair value 144.3 210.0
Securities in an unrealized loss position for more than twelve months, Fair value 39.1  
Total, Fair value $ 183.4 $ 210.0