Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 258,737	$ 249,285	$ 264,076	$ 258,150	$ 240,923	$ 218,632	$ 256,292	$ 231,198	$ 1,030,248	$ 947,045	$ 825,465
Gross profit	57,571	51,596	56,152	59,317	48,871	44,598	56,052	50,173	224,636	199,694	152,532
Income before income taxes	26,762	21,773	28,430	31,960	20,849	18,683	28,533	23,721	108,925	91,786	54,387
Net income	$ 17,348	$ 14,553	$ 17,637	$ 21,661	$ 13,372	$ 12,013	$ 18,180	$ 15,158	$ 71,199	$ 58,723	$ 35,499
Earnings per share
Basic (usd per share)	$ 1.07	$ 0.90	$ 1.08	$ 1.33	$ 0.82	$ 0.74	$ 1.12	$ 0.94	$ 4.38	$ 3.61	$ 2.25
Diluted (usd per share)	$ 1.06	$ 0.89	$ 1.07	$ 1.32	$ 0.81	$ 0.73	$ 1.10	$ 0.92	$ 4.34	$ 3.57	$ 2.21