Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Aug. 31, 2018	May 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable, allowances	$ 2,222	$ 2,466
Preferred stock, par value (in usd per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	75,000,000	75,000,000
Common stock, shares issued (in shares)	37,861,057	37,594,493
Common stock, shares outstanding (in shares)	37,491,057	37,224,493
Treasury stock, shares (in shares)	370,000	370,000