Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2012	Nov. 30, 2011	Nov. 30, 2012	Nov. 30, 2011
Basic Earnings per Share
Net income	$ 78,027	$ 74,350	$ 154,760	$ 142,988
Less dividends to:
Dividends	79,744	70,820	79,744	70,820
Undistributed net (loss) income	(1,717)	3,530	75,016	72,168
Less: net (loss) income allocated to participating unvested securities	(8)	21	409	448
Net (loss) income available to common shareholders	(1,709)	3,509	74,607	71,720
Basic weighted average common shares outstanding (in shares)	124,185	129,727	125,153	130,522
Basic earnings per common share:
Total common shares (in dollars per share)	$ 0.63	$ 0.57	$ 1.24	$ 1.09
Diluted Earnings per Share
Net income	78,027	74,350	154,760	142,988
Less dividends to:
Dividends	79,744	70,820	79,744	70,820
Undistributed Earnings, Diluted	(1,717)	3,530	75,016	72,168
Less: net (loss) income allocated to participating unvested securities	(8)	21	409	448
Net (loss) income available to common shareholders	(1,709)	3,509	74,607	71,720
Basic weighted average common shares outstanding (in shares)	124,185	129,727	125,153	130,522
Effect of dilutive securities - employee stock options (in shares)	424	13	388	21
Diluted weighted average common shares outstanding (in shares)	124,609	129,740	125,541	130,543
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Diluted earnings per share (in dollars per share)	$ 0.63	$ 0.57	$ 1.23	$ 1.09
Common Stock [Member]
Less dividends to:
Dividends	78,866	70,055	78,866	70,055
Basic earnings per common share:
Basic earnings per share - distributed earnings (in dollars per share)	$ 0.64	$ 0.54	$ 0.64	$ 0.54
Earnings per share, basic - undistributed earnings (in dollars per share)	$ (0.01)	$ 0.03	$ 0.60	$ 0.55
Total common shares (in dollars per share)	$ 0.63	$ 0.57	$ 1.24	$ 1.09
Less dividends to:
Dividends	78,866	70,055	78,866	70,055
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Earnings per share, diluted, distributed (in dollars per share)	$ 0.64	$ 0.54	$ 0.64	$ 0.54
Earnings per share, diluted, undistributed (in dollars per share)	$ (0.01)	$ 0.03	$ 0.59	$ 0.55
Nonvested Stock [Member]
Less dividends to:
Dividends	878	765	878	765
Basic earnings per common share:
Basic earnings per share - distributed earnings (in dollars per share)	$ 0.64	$ 0.54	$ 0.64	$ 0.54
Earnings per share, basic - undistributed earnings (in dollars per share)	$ (0.01)	$ 0.03	$ 0.60	$ 0.55
Total common shares (in dollars per share)	$ 0.63	$ 0.57	$ 1.24	$ 1.09
Less dividends to:
Dividends	$ 878	$ 765	$ 878	$ 765
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Earnings per share, diluted, distributed (in dollars per share)	$ 0.64	$ 0.54	$ 0.64	$ 0.54
Earnings per share, diluted, undistributed (in dollars per share)	$ (0.01)	$ 0.03	$ 0.59	$ 0.55
Diluted earnings per share (in dollars per share)	$ 0.63	$ 0.57	$ 1.23	$ 1.09