Funds Held for Clients and Corporate Investments (Funds Held for Clients and Corporate Investments) (Details) - USD ($) $ in Millions		May 31, 2016	May 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Funds held for clients money market securities and other cash equivalents, Amortized cost		$ 502.4	$ 1,430.0
Funds held for clients money market securities and other cash equivalents, Gross unrealized gains
Funds held for clients money market securities and other cash equivalents, Gross unrealized losses
Funds held for clients money market securities and other cash equivalents, Fair value		502.4	1,430.0
Available-for-sale securities, Amortized cost		4,094.3	3,582.0
Available-for-sale securities, Gross unrealized gains		48.3	21.1
Available-for-sale securities, Gross unrealized losses		(0.7)	(7.5)
Available-for-sale securities, Fair value		4,141.9	3,595.6
Other, Amortized cost		14.2	12.7
Other, Gross unrealized gains		0.8	1.5
Other, Gross unrealized losses		(0.1)
Other, Fair value		14.9	14.2
Total funds held for clients and corporate investments, Amortized cost		4,610.9	5,024.7
Total funds held for clients and corporate investments, Gross unrealized gains		49.1	22.6
Total funds held for clients and corporate investments, Gross unrealized losses		(0.8)	(7.5)
Total funds held for clients and corporate investments, Fair value		4,659.2	5,039.8
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost		128.4
Available-for-sale securities, Gross unrealized gains		2.9
Available-for-sale securities, Gross unrealized losses		(0.1)
Available-for-sale securities, Fair value		131.2
General Obligation Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost		1,594.1	1,694.0
Available-for-sale securities, Gross unrealized gains		27.6	14.0
Available-for-sale securities, Gross unrealized losses		(0.1)	(4.3)
Available-for-sale securities, Fair value		1,621.6	1,703.7
Pre-Refunded Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost	[1]	60.2	101.7
Available-for-sale securities, Gross unrealized gains	[1]	1.4	1.0
Available-for-sale securities, Gross unrealized losses	[1]
Available-for-sale securities, Fair value	[1]	61.6	102.7
Revenue Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost		916.2	960.7
Available-for-sale securities, Gross unrealized gains		15.8	6.1
Available-for-sale securities, Gross unrealized losses		(0.2)	(3.2)
Available-for-sale securities, Fair value		931.8	963.6
U.S. Government Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost		160.8
Available-for-sale securities, Gross unrealized gains		0.6
Available-for-sale securities, Gross unrealized losses		(0.3)
Available-for-sale securities, Fair value		161.1
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Amortized cost		1,234.6	825.6
Available-for-sale securities, Gross unrealized gains
Available-for-sale securities, Gross unrealized losses
Available-for-sale securities, Fair value		$ 1,234.6	$ 825.6

[1] Pre-refunded municipal bonds are secured by an escrow fund of U.S. government obligations.