Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
Aug. 31, 2018
May 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses $ (25.5) $ (27.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (13.3) (13.2)
Total, Gross unrealized losses (38.8) (40.7)
Securities in an unrealized loss position for less than twelve months, Fair value 1,708.7 1,907.0
Securities in an unrealized loss position for more than twelve months, Fair value 336.6 331.3
Total, Fair value 2,045.3 2,238.3
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 (5.8) (6.0)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.5) (1.3)
Total, Gross unrealized losses (7.3) (7.3)
Securities in an unrealized loss position for less than twelve months, Fair value 272.9 271.5
Securities in an unrealized loss position for more than twelve months, Fair value 39.7 34.6
Total, Fair value 312.6 306.1
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 (7.4) (8.7)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (2.3) (2.5)
Total, Gross unrealized losses (9.7) (11.2)
Securities in an unrealized loss position for less than twelve months, Fair value 745.1 856.4
Securities in an unrealized loss position for more than twelve months, Fair value 64.1 68.9
Total, Fair value 809.2 925.3
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) (0.1)
Total, Gross unrealized losses (0.1) (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 12.8 18.5
Securities in an unrealized loss position for more than twelve months, Fair value 0.5 0.5
Total, Fair value 13.3 19.0
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 (5.2) (6.3)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.9) (1.7)
Total, Gross unrealized losses (7.1) (8.0)
Securities in an unrealized loss position for less than twelve months, Fair value 445.1 540.9
Securities in an unrealized loss position for more than twelve months, Fair value 55.6 50.9
Total, Fair value 500.7 591.8
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 (7.0) (6.4)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (7.6) (7.7)
Total, Gross unrealized losses (14.6) (14.1)
Securities in an unrealized loss position for less than twelve months, Fair value 232.8 219.7
Securities in an unrealized loss position for more than twelve months, Fair value 176.7 176.4
Total, Fair value $ 409.5 $ 396.1