Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Feb. 28, 2013	Feb. 29, 2012	Feb. 28, 2013	Feb. 29, 2012
Basic Earnings per Share
Net income	$ 74,705	$ 76,035	$ 229,465	$ 219,023
Less dividends to:
Dividends	0	0	79,744	70,820
Undistributed net income	74,705	76,035	149,721	148,203
Less: net income allocated to participating unvested securities	517	556	1,048	1,096
Net income available to common shareholders	74,188	75,479	148,673	147,107
Basic weighted average common shares outstanding (in shares)	123,120	129,735	124,483	130,261
Basic earnings per common share:
Total common shares (in dollars per share)	$ 0.60	$ 0.58	$ 1.84	$ 1.67
Diluted Earnings per Share
Net income	74,705	76,035	229,465	219,023
Less dividends to:
Dividends	0	0	79,744	70,820
Undistributed Earnings, Diluted	74,705	76,035	149,721	148,203
Less: net income allocated to participating unvested securities	517	556	1,048	1,096
Net income available to common shareholders	74,188	75,479	148,673	147,107
Basic weighted average common shares outstanding (in shares)	123,120	129,735	124,483	130,261
Effect of dilutive securities - employee stock options (in shares)	637	210	418	60
Diluted weighted average common shares outstanding (in shares)	123,757	129,945	124,901	130,321
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Diluted earnings per share (in dollars per share)	$ 0.60	$ 0.58	$ 1.83	$ 1.67
Common Stock [Member]
Less dividends to:
Dividends	0	0	78,866	70,055
Basic earnings per common share:
Basic earnings per share - distributed earnings (in dollars per share)	$ 0.00	$ 0.00	$ 0.64	$ 0.54
Earnings per share, basic - undistributed earnings (in dollars per share)	$ 0.60	$ 0.58	$ 1.20	$ 1.13
Total common shares (in dollars per share)	$ 0.60	$ 0.58	$ 1.84	$ 1.67
Less dividends to:
Dividends	0	0	78,866	70,055
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Earnings per share, diluted, distributed (in dollars per share)	$ 0.00	$ 0.00	$ 0.64	$ 0.54
Earnings per share, diluted, undistributed (in dollars per share)	$ 0.60	$ 0.58	$ 1.19	$ 1.13
Nonvested Stock [Member]
Less dividends to:
Dividends	0	0	878	765
Basic earnings per common share:
Basic earnings per share - distributed earnings (in dollars per share)	$ 0.00	$ 0.00	$ 0.64	$ 0.54
Earnings per share, basic - undistributed earnings (in dollars per share)	$ 0.60	$ 0.58	$ 1.20	$ 1.13
Total common shares (in dollars per share)	$ 0.60	$ 0.58	$ 1.84	$ 1.67
Less dividends to:
Dividends	$ 0	$ 0	$ 878	$ 765
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Earnings per share, diluted, distributed (in dollars per share)	$ 0.00	$ 0.00	$ 0.64	$ 0.54
Earnings per share, diluted, undistributed (in dollars per share)	$ 0.60	$ 0.58	$ 1.19	$ 1.13
Diluted earnings per share (in dollars per share)	$ 0.60	$ 0.58	$ 1.83	$ 1.67