Consolidated Statements of Stockholders' Equity (USD $) In Millions, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balance at May. 31, 2011	$ 1,496.2	$ 3.6	$ 535.6	$ 919.5	$ 37.5
Beginning balance, shares at May. 31, 2011		362.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	548.0			548.0
Unrealized gains/(losses) on securities, net of tax	0.2				0.2
Cash dividends declared	(460.5)			(460.5)
Stock-based compensation	23.1		23.1
Stock-based award transactions, Shares		0.5
Stock-based award transactions, Value	(2.5)		(2.4)	(4.9)
Ending balance at May. 31, 2012	1,604.5	3.6	561.1	1,002.1	37.7
Ending balance, Shares at May. 31, 2012		362.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	569.0			569.0
Unrealized gains/(losses) on securities, net of tax	(15.7)				(15.7)
Cash dividends declared	(476.7)			(476.7)
Stock-based compensation	22.9		22.9
Stock-based award transactions, Shares		2.8
Stock-based award transactions, Value	(69.7)	(0.1)	(75.5)	(5.9)
Ending balance at May. 31, 2013	1,773.7	3.7	659.5	1,088.5	22.0
Ending balance, Shares at May. 31, 2013	365.4	365.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	627.5			627.5
Unrealized gains/(losses) on securities, net of tax	(0.5)				(0.5)
Cash dividends declared	(510.6)			(510.6)
Repurchases of common shares, Shares		(6.2)
Repurchase of common shares, Value	(249.7)	(0.1)	(11.2)	(238.4)
Stock-based compensation	26.4		26.4
Stock-based award transactions, Shares		3.8
Stock-based award transactions, Value	(110.2)		(119.7)	(9.5)
Ending balance at May. 31, 2014	$ 1,777.0	$ 3.6	$ 794.4	$ 957.5	$ 21.5
Ending balance, Shares at May. 31, 2014	363.0	363.0