Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Nov. 30, 2015	May. 31, 2015
Statement Of Financial Position [Abstract]
Trade accounts receivable, allowances	$ 25,110	$ 24,526
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, authorized	50,000,000	50,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized	300,000,000	300,000,000
Common stock, issued	139,979,000	138,828,000
Common stock, outstanding	133,318,000	133,203,000