Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						6 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jul. 31, 2016	Oct. 31, 2016	Jan. 31, 2017	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 405,887	$ 292,791	$ 274,769	$ 276,827	$ 258,737	$ 258,150	$ 264,076	$ 249,285	$ 1,250,274	$ 1,030,248	$ 947,045
Gross profit	89,195	50,379	57,335	58,494	57,571	59,317	56,152	51,596	255,403	224,636	199,694
Income before income taxes	28,161	3,764	31,463	31,372	26,762	31,960	28,430	21,773	94,760	108,925	91,786
Net income	$ 19,109	$ 1,996	$ 19,755	$ 22,281	$ 17,348	$ 21,661	$ 17,637	$ 14,553	$ 63,141	$ 71,199	$ 58,723
Earnings per share
Basic (usd per share)	$ 1.09	$ 0.12	$ 1.22	$ 1.37	$ 1.07	$ 1.33	$ 1.08	$ 0.90	$ 3.80	$ 4.38	$ 3.61
Diluted (usd per share)	$ 1.08	$ 0.12	$ 1.21	$ 1.36	$ 1.06	$ 1.32	$ 1.07	$ 0.89	$ 3.77	$ 4.34	$ 3.57