Quarterly report pursuant to Section 13 or 15(d)

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

v3.19.1
Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
Feb. 28, 2019
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 $ (0.6) $ (27.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (17.2) (13.2)
Total, Gross unrealized losses (17.8) (40.7)
Securities in an unrealized loss position for less than twelve months, Fair value 134.0 1,907.0
Securities in an unrealized loss position for more than twelve months, Fair value 1,319.6 331.3
Total, Fair value 1,453.6 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 (0.1) (6.0)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (4.2) (1.3)
Total, Gross unrealized losses (4.3) (7.3)
Securities in an unrealized loss position for less than twelve months, Fair value 23.0 271.5
Securities in an unrealized loss position for more than twelve months, Fair value 262.4 34.6
Total, Fair value 285.4 306.1
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 (0.1) (8.7)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (2.5) (2.5)
Total, Gross unrealized losses (2.6) (11.2)
Securities in an unrealized loss position for less than twelve months, Fair value 56.2 856.4
Securities in an unrealized loss position for more than twelve months, Fair value 397.8 68.9
Total, Fair value 454.0 925.3
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)
Total, Gross unrealized losses   (0.1)
Securities in an unrealized loss position for less than twelve months, Fair value 0.1 18.5
Securities in an unrealized loss position for more than twelve months, Fair value 5.5 0.5
Total, Fair value 5.6 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   (6.3)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (1.9) (1.7)
Total, Gross unrealized losses (1.9) (8.0)
Securities in an unrealized loss position for less than twelve months, Fair value 13.2 540.9
Securities in an unrealized loss position for more than twelve months, Fair value 273.5 50.9
Total, Fair value 286.7 591.8
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 (0.4) (6.4)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (8.6) (7.7)
Total, Gross unrealized losses (9.0) (14.1)
Securities in an unrealized loss position for less than twelve months, Fair value 41.5 219.7
Securities in an unrealized loss position for more than twelve months, Fair value 380.4 176.4
Total, Fair value $ 421.9 $ 396.1