Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Feb. 28, 2018	May 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses	$ (26.2)	$ (5.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(13.7)
Total, Gross unrealized losses	(39.9)	(5.8)
Securities in an unrealized loss position for less than twelve months, Fair value	1,649.8	606.4
Securities in an unrealized loss position for more than twelve months, Fair value	334.3	1.0
Total, Fair value	1,984.1	607.4
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	(5.2)	(0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(1.3)
Total, Gross unrealized losses	(6.5)	(0.5)
Securities in an unrealized loss position for less than twelve months, Fair value	252.1	43.6
Securities in an unrealized loss position for more than twelve months, Fair value	34.7
Total, Fair value	286.8	43.6
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	(8.0)	(0.9)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(2.6)
Total, Gross unrealized losses	(10.6)	(0.9)
Securities in an unrealized loss position for less than twelve months, Fair value	735.5	188.8
Securities in an unrealized loss position for more than twelve months, Fair value	71.4
Total, Fair value	806.9	188.8
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	7.2	9.2
Securities in an unrealized loss position for more than twelve months, Fair value	0.5
Total, Fair value	7.7	9.2
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	(6.1)	(0.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(1.8)
Total, Gross unrealized losses	(7.9)	(0.8)
Securities in an unrealized loss position for less than twelve months, Fair value	435.7	154.8
Securities in an unrealized loss position for more than twelve months, Fair value	51.6	1.0
Total, Fair value	487.3	155.8
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	(6.8)	(3.6)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(8.0)
Total, Gross unrealized losses	(14.8)	(3.6)
Securities in an unrealized loss position for less than twelve months, Fair value	219.3	210.0
Securities in an unrealized loss position for more than twelve months, Fair value	176.1
Total, Fair value	$ 395.4	$ 210.0