Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2011	Feb. 28, 2011	Nov. 30, 2010	Aug. 31, 2010	May 31, 2012	May 31, 2011	May 31, 2010
Basic Earnings per Share
Net income	$ 78,614	$ 76,035	$ 74,350	$ 68,638	$ 70,776	$ 59,070	$ 55,866	$ 61,277	$ 297,637	$ 246,989	$ 215,620
Less dividends to:
Total dividends									70,820	71,812	73,960
Undistributed net income									226,817	175,177	141,660
Less: net income allocated to participating unvested securities									1,880	1,097	661
Net income available to common shareholders									224,937	174,080	140,999
Basic weighted average common shares outstanding (in shares)	128,788	129,735	129,727	131,309	143,317	145,303	145,511	152,164	129,891	146,586	152,858
Basic earnings per share:
Total common shares (in dollars per share)	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 0.49	$ 0.41	$ 0.38	$ 0.40	$ 2.27	$ 1.68	$ 1.40
Diluted Earnings per Share
Net income	78,614	76,035	74,350	68,638	70,776	59,070	55,866	61,277	297,637	246,989	215,620
Less dividends to:
Total dividends									70,820	71,812	73,960
Undistributed net income									226,817	175,177	141,660
Less: net income allocated to participating unvested securities									1,880	1,097	661
Net income available to common shareholders									224,937	174,080	140,999
Basic weighted average common shares outstanding (in shares)	128,788	129,735	129,727	131,309	143,317	145,303	145,511	152,164	129,891	146,586	152,858
Effect of dilutive securities - employee stock options (in shares)									142	0	0
Diluted weighted average common shares outstanding (in shares)									130,033	146,586	152,858
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 0.49	$ 0.41	$ 0.38	$ 0.40	$ 2.27	$ 1.68	$ 1.40
Common Stock [Member]
Less dividends to:
Total dividends									70,055	71,197	73,377
Basic earnings per share:
Earnings per share, basic, distributed (in dollars per share)									$ 0.54	$ 0.49	$ 0.48
Earnings per share, basic, undistributed (in dollars per share)									$ 1.73	$ 1.19	$ 0.92
Total common shares (in dollars per share)									$ 2.27	$ 1.68	$ 1.40
Less dividends to:
Total dividends									70,055	71,197	73,377
Diluted earnings per share:
Earnings per share, diluted, distributed (in dollars per share)									$ 0.54	$ 0.49	$ 0.48
Earnings per share, diluted, undistributed (in dollars per share)									$ 1.73	$ 1.19	$ 0.92
Diluted earnings per share (in dollars per share)									$ 2.27	$ 1.68	$ 1.40
Unvested shares [Member]
Less dividends to:
Total dividends									765	615	583
Basic earnings per share:
Earnings per share, basic, distributed (in dollars per share)									$ 0.54	$ 0.49	$ 0.48
Earnings per share, basic, undistributed (in dollars per share)									$ 1.73	$ 1.19	$ 0.92
Total common shares (in dollars per share)									$ 2.27	$ 1.68	$ 1.40
Less dividends to:
Total dividends									$ 765	$ 615	$ 583
Diluted earnings per share:
Earnings per share, diluted, distributed (in dollars per share)									$ 0.54	$ 0.49	$ 0.48
Earnings per share, diluted, undistributed (in dollars per share)									$ 1.73	$ 1.19	$ 0.92
Diluted earnings per share (in dollars per share)									$ 2.27	$ 1.68	$ 1.40