Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 407,399	$ 384,080	$ 424,878	$ 428,962	$ 405,887	$ 292,791	$ 274,769	$ 276,827	$ 1,645,319	$ 1,250,274	$ 1,030,248
Gross profit	87,122	76,853	86,762	95,736	89,195	50,379	57,335	58,494	346,473	255,403	224,636
Income before income taxes	28,814	24,126	25,409	32,539	28,161	3,764	31,463	31,372	110,888	94,760	108,925
Net income	$ 22,024	$ 18,409	$ 18,488	$ 24,767	$ 19,109	$ 1,996	$ 19,755	$ 22,281	$ 83,688	$ 63,141	$ 71,199
Earnings per share
Basic (usd per share)	$ 1.31	$ 1.07	$ 1.05	$ 1.41	$ 1.09	$ 0.12	$ 1.22	$ 1.37	$ 4.84	$ 3.80	$ 4.38
Diluted (usd per share)	$ 1.30	$ 1.07	$ 1.05	$ 1.41	$ 1.08	$ 0.12	$ 1.21	$ 1.36	$ 4.83	$ 3.77	$ 4.34