Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Aug. 31, 2012	Aug. 31, 2011
Basic Earnings per Share
Net income	$ 76,733	$ 68,638
Less: net income allocated to participating unvested securities	365	370
Net income available to common shareholders	76,368	68,268
Basic weighted average common shares outstanding (in shares)	126,110	131,309
Basic earnings per share (in dollars per share)	$ 0.61	$ 0.52
Diluted Earnings per Share
Net income	76,733	68,638
Less: net income allocated to participating unvested securities	365	370
Net income available to common shareholders	$ 76,368	$ 68,268
Basic weighted average common shares outstanding (in shares)	126,110	131,309
Effect of dilutive securities - employee stock options (in shares)	348	29
Diluted weighted average common shares outstanding (in shares)	126,458	131,338
Diluted earnings per share (in dollars per share)	$ 0.60	$ 0.52