Quarterly Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	May. 31, 2015	Feb. 28, 2014	Nov. 30, 2013	Aug. 31, 2013	May. 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May. 31, 2015	May. 31, 2014	May. 31, 2013
Summary of Quarterly Financial Information
Net sales	$ 90,897	$ 86,597	$ 92,149	$ 87,331	$ 94,060	$ 88,150	$ 88,571	$ 83,644	$ 356,974	$ 354,425	$ 341,916
Gross profit	45,557	37,851	47,656	45,825	47,410	44,793	44,885	42,580	176,889	179,668	168,514
Net income	$ (814)	$ (4,262)	$ 1,338	$ 470	$ (1,215)	$ 4,515	$ (261)	$ (373)	$ (3,268)	$ 2,666	$ (1,211)
Earnings per common share
Basic (usd per share)	$ (0.02)	$ (0.12)	$ 0.04	$ 0.01	$ (0.03)	$ 0.13	$ (0.01)	$ (0.01)	$ (0.09)	$ 0.08	$ (0.03)
Diluted (usd per share)	$ (0.02)	$ (0.12)	$ 0.04	$ 0.01	$ (0.03)	$ 0.13	$ (0.01)	$ (0.01)	$ (0.09)	$ 0.08	$ (0.03)