Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($) $ in Millions	Feb. 28, 2017	May 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses	$ (15.2)	$ (0.5)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses		(0.2)
Total, Gross unrealized losses	(15.2)	(0.7)
Securities in an unrealized loss position for less than twelve months, Fair value	782.1	141.1
Securities in an unrealized loss position for more than twelve months, Fair value	1.0	14.5
Total, Fair value	783.1	155.6
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)	(0.1)
Total, Gross unrealized losses	(1.0)	(0.1)
Securities in an unrealized loss position for less than twelve months, Fair value	57.4	14.7
Total, Fair value	57.4	14.7
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	(4.6)	(0.1)
Total, Gross unrealized losses	(4.6)	(0.1)
Securities in an unrealized loss position for less than twelve months, Fair value	258.6	48.9
Securities in an unrealized loss position for more than twelve months, Fair value		2.8
Total, Fair value	258.6	51.7
Pre-Refunded Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Fair value	12.4	5.7
Total, Fair value	12.4	5.7
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	(4.1)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses		(0.2)
Total, Gross unrealized losses	(4.1)	(0.2)
Securities in an unrealized loss position for less than twelve months, Fair value	199.1	20.7
Securities in an unrealized loss position for more than twelve months, Fair value	1.0	11.7
Total, Fair value	200.1	32.4
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	(5.5)	(0.3)
Total, Gross unrealized losses	(5.5)	(0.3)
Securities in an unrealized loss position for less than twelve months, Fair value	254.6	51.1
Total, Fair value	$ 254.6	$ 51.1