Quarterly report pursuant to Section 13 or 15(d)

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

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Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($)
$ in Millions
Nov. 30, 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 $ (10.2) $ (27.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (36.0) (13.2)
Total, Gross unrealized losses (46.2) (40.7)
Securities in an unrealized loss position for less than twelve months, Fair value 995.7 1,907.0
Securities in an unrealized loss position for more than twelve months, Fair value 1,253.9 331.3
Total, Fair value 2,249.6 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 (4.6) (6.0)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (6.1) (1.3)
Total, Gross unrealized losses (10.7) (7.3)
Securities in an unrealized loss position for less than twelve months, Fair value 230.9 271.5
Securities in an unrealized loss position for more than twelve months, Fair value 144.8 34.6
Total, Fair value 375.7 306.1
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 (2.5) (8.7)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (9.2) (2.5)
Total, Gross unrealized losses (11.7) (11.2)
Securities in an unrealized loss position for less than twelve months, Fair value 417.6 856.4
Securities in an unrealized loss position for more than twelve months, Fair value 447.5 68.9
Total, Fair value 865.1 925.3
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 22.1 18.5
Securities in an unrealized loss position for more than twelve months, Fair value 5.6 0.5
Total, Fair value 27.7 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 (1.3) (6.3)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (7.0) (1.7)
Total, Gross unrealized losses (8.3) (8.0)
Securities in an unrealized loss position for less than twelve months, Fair value 213.9 540.9
Securities in an unrealized loss position for more than twelve months, Fair value 308.1 50.9
Total, Fair value 522.0 591.8
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 (1.7) (6.4)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses (13.7) (7.7)
Total, Gross unrealized losses (15.4) (14.1)
Securities in an unrealized loss position for less than twelve months, Fair value 111.2 219.7
Securities in an unrealized loss position for more than twelve months, Fair value 347.9 176.4
Total, Fair value $ 459.1 $ 396.1