Funds Held for Clients and Corporate Investments (Securities in Unrealized Loss Position) (Details) - USD ($) $ in Millions	May 31, 2016	May 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Securities in an unrealized loss position for less than twelve months, Gross unrealized losses	$ (0.5)	$ (7.0)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(0.2)	(0.5)
Total, Gross unrealized losses	(0.7)	(7.5)
Securities in an unrealized loss position for less than twelve months, Fair value	141.1	896.7
Securities in an unrealized loss position for more than twelve months, Fair value	14.5	26.3
Total, Fair value	155.6	923.0
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	(0.1)
Total, Gross unrealized losses	(0.1)
Securities in an unrealized loss position for less than twelve months, Fair value	14.7
Total, Fair value	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	(0.1)	(3.8)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses		(0.5)
Total, Gross unrealized losses	(0.1)	(4.3)
Securities in an unrealized loss position for less than twelve months, Fair value	48.9	535.1
Securities in an unrealized loss position for more than twelve months, Fair value	2.8	26.3
Total, Fair value	51.7	561.4
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	5.7
Total, Fair value	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		(3.2)
Securities in an unrealized loss position for more than twelve months, Gross unrealized losses	(0.2)
Total, Gross unrealized losses	(0.2)	(3.2)
Securities in an unrealized loss position for less than twelve months, Fair value	20.7	361.6
Securities in an unrealized loss position for more than twelve months, Fair value	11.7
Total, Fair value	32.4	$ 361.6
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	(0.3)
Total, Gross unrealized losses	(0.3)
Securities in an unrealized loss position for less than twelve months, Fair value	51.1
Total, Fair value	$ 51.1