Earnings Per Share (Computation of Basic Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Aug. 31, 2013	Aug. 31, 2012
Basic Earnings per Share
Net income	$ 77,754	$ 76,733
Less: net income allocated to participating unvested securities	460	365
Net income available to common shareholders	77,294	76,368
Basic weighted average common shares outstanding (shares)	122,130	126,110
Basic earnings per share (dollars per share)	$ 0.63	$ 0.61
Diluted Earnings per Share
Net income	77,754	76,733
Less: net income allocated to participating unvested securities	460	365
Net income available to common shareholders	$ 77,294	$ 76,368
Basic weighted average common shares outstanding (shares)	122,130	126,110
Effect of dilutive securities - employee stock options (shares)	762	348
Diluted weighted average common shares outstanding (shares)	122,892	126,458
Diluted earnings per share (dollars per share)	$ 0.63	$ 0.60