Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($) $ in Millions	May 31, 2024	May 31, 2023
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses	$ (23.0)	$ (15.2)
Securities in an unrealized loss position for less than twelve months, Fair value	1,068.9	992.5
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(141.2)	(164.2)
Securities in an unrealized loss position for more than twelve months, Fair value	2,102.7	1,752.9
Total, Gross unrealized losses	(164.2)	(179.4)
Total, Fair value	3,171.6	2,745.4
Asset-Backed 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)	(0.1)
Securities in an unrealized loss position for less than twelve months, Fair value	77.4	54.2
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(0.7)	(1.3)
Securities in an unrealized loss position for more than twelve months, Fair value	43.0	23.3
Total, Gross unrealized losses	(1.1)	(1.4)
Total, Fair value	120.4	77.5
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	(8.4)	(5.9)
Securities in an unrealized loss position for less than twelve months, Fair value	620.8	652.0
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(24.4)	(25.2)
Securities in an unrealized loss position for more than twelve months, Fair value	647.8	382.7
Total, Gross unrealized losses	(32.8)	(31.1)
Total, Fair value	1,268.6	1,034.7
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	(8.0)	(5.9)
Securities in an unrealized loss position for less than twelve months, Fair value	102.5	86.7
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(78.9)	(99.4)
Securities in an unrealized loss position for more than twelve months, Fair value	864.7	889.0
Total, Gross unrealized losses	(86.9)	(105.3)
Total, Fair value	967.2	975.7
U.S. Government Agency And Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses	(6.2)	(3.3)
Securities in an unrealized loss position for less than twelve months, Fair value	268.2	199.6
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(37.2)	(38.3)
Securities in an unrealized loss position for more than twelve months, Fair value	547.2	457.9
Total, Gross unrealized losses	(43.4)	(41.6)
Total, Fair value	$ 815.4	$ 657.5