Annual report pursuant to Section 13 and 15(d)

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

v3.7.0.1
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
May 31, 2017
May 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses $ (5.8) $ (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.2)
Total, Gross unrealized losses (5.8) (0.7)
Securities in an unrealized loss position for less than twelve months, Fair value 606.4 141.1
Securities in an unrealized loss position for more than twelve months, Fair value 1.0 14.5
Total, Fair value 607.4 155.6
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 (0.5) (0.1)
Total, Gross unrealized losses (0.5) (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 43.6 14.7
Total, Fair value 43.6 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 (0.9) (0.1)
Total, Gross unrealized losses (0.9) (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 188.8 48.9
Securities in an unrealized loss position for more than twelve months, Fair value   2.8
Total, Fair value 188.8 51.7
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 9.2 5.7
Total, Fair value 9.2 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 (0.8)  
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.2)
Total, Gross unrealized losses (0.8) (0.2)
Securities in an unrealized loss position for less than twelve months, Fair value 154.8 20.7
Securities in an unrealized loss position for more than twelve months, Fair value 1.0 11.7
Total, Fair value 155.8 32.4
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 (3.6) (0.3)
Total, Gross unrealized losses (3.6) (0.3)
Securities in an unrealized loss position for less than twelve months, Fair value 210.0 51.1
Total, Fair value $ 210.0 $ 51.1