Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Feb. 28, 2021	May 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses	$ (4.4)	$ (1.7)
Total, Gross unrealized losses	(4.4)	(1.7)
Securities in an unrealized loss position for less than twelve months, Fair value	435.2	66.8
Total, Fair value	435.2	66.8
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Fair value	7.1
Total, Fair value	7.1
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	(0.2)	(0.1)
Total, Gross unrealized losses	(0.2)	(0.1)
Securities in an unrealized loss position for less than twelve months, Fair value	22.6	6.5
Total, Fair value	22.6	6.5
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	(2.9)	(1.6)
Total, Gross unrealized losses	(2.9)	(1.6)
Securities in an unrealized loss position for less than twelve months, Fair value	300.7	60.3
Total, Fair value	300.7	$ 60.3
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	(1.3)
Total, Gross unrealized losses	(1.3)
Securities in an unrealized loss position for less than twelve months, Fair value	104.8
Total, Fair value	$ 104.8