Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2013	Nov. 30, 2012	Nov. 30, 2013	Nov. 30, 2012
Basic Earnings per Share
Net income	$ 84,862	$ 78,027	$ 162,616	$ 154,760
Less dividends to:
Dividends	93,320	79,744	93,320	79,744
Undistributed net (loss) income	(8,458)	(1,717)	69,296	75,016
Less: net (loss) income allocated to participating unvested securities	(59)	(8)	482	409
Net (loss) income available to common shareholders	(8,399)	(1,709)	68,814	74,607
Basic weighted average common shares outstanding (shares)	119,907	124,185	121,025	125,153
Basic earnings per common share:
Total common shares, basic (in dollars per share)	$ 0.71	$ 0.63	$ 1.34	$ 1.24
Diluted Earnings per Share
Net income	84,862	78,027	162,616	154,760
Less dividends to:
Dividends	93,320	79,744	93,320	79,744
Undistributed net (loss) income	(8,458)	(1,717)	69,296	75,016
Less: net (loss) income allocated to participating unvested securities	(59)	(8)	482	409
Net (loss) income available to common shareholders	(8,399)	(1,709)	68,814	74,607
Basic weighted average common shares outstanding (shares)	119,907	124,185	121,025	125,153
Effect of dilutive securities - employee stock options (shares)	1,143	424	991	388
Diluted weighted average common shares outstanding (shares)	121,050	124,609	122,016	125,541
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Total common shares, diluted (in dollars per share)	$ 0.70	$ 0.63	$ 1.33	$ 1.23
Common Stock
Less dividends to:
Dividends	92,229	78,866	92,229	78,866
Basic earnings per common share:
Earnings per share, basic - distributed earnings (in dollars per share)	$ 0.77	$ 0.64	$ 0.77	$ 0.64
Earnings per share, basic - undistributed earnings (in dollars per share)	$ (0.06)	$ (0.01)	$ 0.57	$ 0.60
Total common shares, basic (in dollars per share)	$ 0.71	$ 0.63	$ 1.34	$ 1.24
Less dividends to:
Dividends	92,229	78,866	92,229	78,866
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Earnings per share, diluted - distributed earnings (in dollars per share)	$ 0.77	$ 0.64	$ 0.77	$ 0.64
Earnings per share, diluted - undistributed earnings (in dollars per share)	$ (0.07)	$ (0.01)	$ 0.56	$ 0.59
Total common shares, diluted (in dollars per share)	$ 0.70	$ 0.63	$ 1.33	$ 1.23
Nonvested Stock
Less dividends to:
Dividends	1,091	878	1,091	878
Basic earnings per common share:
Earnings per share, basic - distributed earnings (in dollars per share)	$ 0.77	$ 0.64	$ 0.77	$ 0.64
Earnings per share, basic - undistributed earnings (in dollars per share)	$ (0.06)	$ (0.01)	$ 0.57	$ 0.60
Total common shares, basic (in dollars per share)	$ 0.71	$ 0.63	$ 1.34	$ 1.24
Less dividends to:
Dividends	$ 1,091	$ 878	$ 1,091	$ 878
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Earnings per share, diluted - distributed earnings (in dollars per share)	$ 0.77	$ 0.64	$ 0.77	$ 0.64
Earnings per share, diluted - undistributed earnings (in dollars per share)	$ (0.07)	$ (0.01)	$ 0.56	$ 0.59
Total common shares, diluted (in dollars per share)	$ 0.70	$ 0.63	$ 1.33	$ 1.23