Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	May. 31, 2015	May. 31, 2014
Statement of Financial Position [Abstract]
Allowance on accounts receivable	$ 15,674	$ 14,906
Preferred stock, shares authorized (in shares)	100,000	100,000
Preferred stock, shares outstanding ( in shares)	0	0
Common stock, shares authorized (in shares)	425,000,000	425,000,000
Common stock, shares issued (in shares)	178,117,334	176,378,412
Common stock, shares outstanding (in shares)	111,702,949	117,037,784
Treasury stock, shares (in shares)	66,414,385	59,340,628