Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Beginning balance, shares at May. 31, 2012		362.6
Beginning balance at May. 31, 2012	$ 1,604.5	$ 3.6	$ 561.1	$ 1,002.1	$ 37.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	569.0			569.0
Unrealized 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, Shares at May. 31, 2013		365.4
Ending balance at May. 31, 2013	1,773.7	$ 3.7	659.5	1,088.5	22.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	627.5			627.5
Unrealized 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, Shares at May. 31, 2014	363.0	363.0
Ending balance at May. 31, 2014	$ 1,777.0	$ 3.6	794.4	957.5	21.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	674.9			674.9
Unrealized losses on securities, net of tax	(14.0)				(14.0)
Cash dividends declared	(551.8)			(551.8)
Repurchases of common shares, Shares		(3.9)
Repurchase of common shares, Value	(182.4)		(7.2)	(175.2)
Stock-based compensation	31.5		31.5
Stock-based award transactions, Shares		2.1
Stock-based award transactions, Value	$ 50.3		61.4	(11.1)
Ending balance, Shares at May. 31, 2015	361.2	361.2
Ending balance at May. 31, 2015	$ 1,785.5	$ 3.6	$ 880.1	$ 894.3	$ 7.5