Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Nov. 30, 2017	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	$ (12.9)	$ (5.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(9.8)
Total, Gross unrealized losses	(22.7)	(5.8)
Securities in an unrealized loss position for less than twelve months, Fair value	1,522.5	606.4
Securities in an unrealized loss position for more than twelve months, Fair value	348.3	1.0
Total, Fair value	1,870.8	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	(1.0)	(0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(0.7)
Total, Gross unrealized losses	(1.7)	(0.5)
Securities in an unrealized loss position for less than twelve months, Fair value	117.1	43.6
Securities in an unrealized loss position for more than twelve months, Fair value	35.3
Total, Fair value	152.4	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	(5.5)	(0.9)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(2.1)
Total, Gross unrealized losses	(7.6)	(0.9)
Securities in an unrealized loss position for less than twelve months, Fair value	723.2	188.8
Securities in an unrealized loss position for more than twelve months, Fair value	76.3
Total, Fair value	799.5	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	13.4	9.2
Securities in an unrealized loss position for more than twelve months, Fair value	5.8
Total, Fair value	19.2	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	(4.3)	(0.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(1.4)
Total, Gross unrealized losses	(5.7)	(0.8)
Securities in an unrealized loss position for less than twelve months, Fair value	478.0	154.8
Securities in an unrealized loss position for more than twelve months, Fair value	52.4	1.0
Total, Fair value	530.4	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	(2.0)	(3.6)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(5.6)
Total, Gross unrealized losses	(7.6)	(3.6)
Securities in an unrealized loss position for less than twelve months, Fair value	190.8	210.0
Securities in an unrealized loss position for more than twelve months, Fair value	178.5
Total, Fair value	$ 369.3	$ 210.0