Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	Aug. 31, 2014	May 31, 2014
Trade accounts receivable, allowances	$ 29,197	$ 27,641
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, authorized	50,000	50,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized	300,000	300,000
Common stock, issued	138,386	138,039
Common stock, outstanding	133,511	133,273