Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Aug. 31, 2015	Aug. 31, 2014
Basic Earnings per Share from Continuing Operations
Income from continuing operations	$ 106,198	$ 105,905
Less: income from continuing operations allocated to participating securities	1,742	591
Income from continuing operations available to common shareholders	$ 104,456	$ 105,314
Basic weighted average common shares outstanding (shares)	110,597	116,659
Basic earnings per common share:
Basic earnings per share, continuing operations (dollars per share)	$ 0.94	$ 0.90
Diluted Earnings per Share from Continuing Operations
Income from continuing operations	$ 106,198	$ 105,905
Less: income from continuing operations allocated to participating securities	1,742	591
Income from continuing operations available to common shareholders	$ 104,456	$ 105,314
Basic weighted average common shares outstanding (shares)	110,597	116,659
Effect of dilutive securities - employee stock options (shares)	1,632	1,371
Diluted weighted average common shares outstanding (shares)	112,229	118,030
Diluted earnings per share:
Diluted earnings per share, continuing operations (dollars per share)	$ 0.93	$ 0.89