Quarterly report pursuant to Section 13 or 15(d)

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

v3.5.0.2
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
Aug. 31, 2016
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 $ (0.5) $ (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.2)
Total, Gross unrealized losses (0.5) (0.7)
Securities in an unrealized loss position for less than twelve months, Fair value 160.3 141.1
Securities in an unrealized loss position for more than twelve months, Fair value 3.9 14.5
Total, Fair value 164.2 155.6
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   (0.1)
Total, Gross unrealized losses   (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 8.5 14.7
Total, Fair value 8.5 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 (0.1) (0.1)
Total, Gross unrealized losses (0.1) (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 39.9 48.9
Securities in an unrealized loss position for more than twelve months, Fair value 2.8 2.8
Total, Fair value 42.7 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   5.7
Total, Fair value   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 (0.1)  
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.2)
Total, Gross unrealized losses (0.1) (0.2)
Securities in an unrealized loss position for less than twelve months, Fair value 22.6 20.7
Securities in an unrealized loss position for more than twelve months, Fair value 1.1 11.7
Total, Fair value 23.7 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 (0.3) (0.3)
Total, Gross unrealized losses (0.3) (0.3)
Securities in an unrealized loss position for less than twelve months, Fair value 89.3 51.1
Total, Fair value $ 89.3 $ 51.1