Quarterly report pursuant to Section 13 or 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
Feb. 28, 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 $ (15.2) $ (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.2)
Total, Gross unrealized losses (15.2) (0.7)
Securities in an unrealized loss position for less than twelve months, Fair value 782.1 141.1
Securities in an unrealized loss position for more than twelve months, Fair value 1.0 14.5
Total, Fair value 783.1 155.6
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 (1.0) (0.1)
Total, Gross unrealized losses (1.0) (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 57.4 14.7
Total, Fair value 57.4 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 (4.6) (0.1)
Total, Gross unrealized losses (4.6) (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 258.6 48.9
Securities in an unrealized loss position for more than twelve months, Fair value   2.8
Total, Fair value 258.6 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 12.4 5.7
Total, Fair value 12.4 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 (4.1)  
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses   (0.2)
Total, Gross unrealized losses (4.1) (0.2)
Securities in an unrealized loss position for less than twelve months, Fair value 199.1 20.7
Securities in an unrealized loss position for more than twelve months, Fair value 1.0 11.7
Total, Fair value 200.1 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 (5.5) (0.3)
Total, Gross unrealized losses (5.5) (0.3)
Securities in an unrealized loss position for less than twelve months, Fair value 254.6 51.1
Total, Fair value $ 254.6 $ 51.1