Earnings per Share (Computation of Basic Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2013	May 31, 2012	May 31, 2011
Basic Earnings per Share
Net income	$ 85,977	$ 74,705	$ 78,027	$ 76,733	$ 78,614	$ 76,035	$ 74,350	$ 68,638	$ 315,442	$ 297,637	$ 246,989
Less dividends to:
Total dividends									79,744	70,820	71,812
Undistributed net income									235,698	226,817	175,177
Less: net income allocated to participating unvested securities									1,896	1,880	1,097
Net income available to common shareholders									233,802	224,937	174,080
Basic weighted average common shares outstanding (shares)	122,392	123,120	124,185	126,110	128,788	129,735	129,727	131,309	123,956	129,891	146,586
Basic earnings per share:
Total common shares (dollars per share)	$ 0.69	$ 0.60	$ 0.63	$ 0.61	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 2.53	$ 2.27	$ 1.68
Common Stock [Member]
Less dividends to:
Total dividends									78,866	70,055	71,197
Basic earnings per share:
Earnings per share, basic, distributed (dollars per share)									$ 0.64	$ 0.54	$ 0.49
Earnings per share, basic, undistributed (dollars per share)									$ 1.89	$ 1.73	$ 1.19
Total common shares (dollars per share)									$ 2.53	$ 2.27	$ 1.68
Unvested shares [Member]
Less dividends to:
Total dividends									$ 878	$ 765	$ 615
Basic earnings per share:
Earnings per share, basic, distributed (dollars per share)									$ 0.64	$ 0.54	$ 0.49
Earnings per share, basic, undistributed (dollars per share)									$ 1.89	$ 1.73	$ 1.19
Total common shares (dollars per share)									$ 2.53	$ 2.27	$ 1.68