Funds Held for Clients and Corporate Investments (Details) (USD $) In Millions, unless otherwise specified	Feb. 28, 2014		May 31, 2013
Type of issue:
Funds held for clients money market securities and other cash equivalents, Amortized cost	$ 2,507.4		$ 1,137.7
Funds held for clients money market securities and other cash equivalents, Gross unrealized gains	0		0
Funds held for clients money market securities and other cash equivalents, Gross unrealized losses	0		0
Funds held for clients money market securities and other cash equivalents, Fair value	2,507.4		1,137.7
Available-for-sale securities:
Amortized cost	3,050.3		3,656.7
Gross unrealized gains	43.1		40.4
Gross unreailzed losses	(3.9)		(5.7)
Fair value	3,089.5		3,691.4
Other, Amortized cost	10.4		9.5
Other, Gross unrealized gains	1.7		1.2
Other, Gross unrealized losses	0		0
Other, Fair value	12.1		10.7
Total funds held for clients and corporate investments, Amortized cost	5,568.1		4,803.9
Total funds held for clients and corporate investments, Gross unrealized gains	44.8		41.6
Total funds held for clients and corporate investments, Gross unrealized losses	(3.9)		(5.7)
Total funds held for clients and corporate investments, Fair value	5,609.0		4,839.8
General obligation municipal bonds [Member]
Available-for-sale securities:
Amortized cost	1,623.2		1,432.9
Gross unrealized gains	28.8		27.4
Gross unreailzed losses	(2.5)		(3.5)
Fair value	1,649.5		1,456.8
Pre-refunded municipal bonds [Member]
Available-for-sale securities:
Amortized cost	124.1	[1]	201.0	[1]
Gross unrealized gains	2.1	[1]	2.9	[1]
Gross unreailzed losses	0	[1]	0	[1]
Fair value	126.2	[1]	203.9	[1]
Revenue municipal bonds [Member]
Available-for-sale securities:
Amortized cost	837.5		746.1
Gross unrealized gains	12.2		10.1
Gross unreailzed losses	(1.4)		(2.2)
Fair value	848.3		754.0
Variable rate demand notes [Member]
Available-for-sale securities:
Amortized cost	465.5		1,276.7
Gross unrealized gains	0		0
Gross unreailzed losses	0		0
Fair value	$ 465.5		$ 1,276.7

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