Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	COMMON STOCK	RETAINED EARNINGS	ACCUMULATED OTHER COMPREHENSIVE LOSS
Balance at beginning of period (shares) at Apr. 30, 2019		16,849,026
Balance at beginning of period at Apr. 30, 2019	$ 620,353	$ 352,424	$ 317,420	$ (49,491)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,881		26,881
Other comprehensive loss, net of tax	315			315
Stock-based compensation	897	$ 897
Exercise of stock-based compensation awards, net of amounts withheld for taxes (shares)		20,923
Exercise of stock-based compensation awards, net of amounts withheld for taxes	(1,050)	$ (1,050)
Employee benefit plan contributions (shares)		45,721
Employee benefit plan contributions	3,772	$ 3,772
Balance at end of period (shares) at Jul. 31, 2019		16,915,670
Balance at end of period at Jul. 31, 2019	651,168	$ 356,043	344,301	(49,176)
Balance at beginning of period (shares) at Apr. 30, 2019		16,849,026
Balance at beginning of period at Apr. 30, 2019	620,353	$ 352,424	317,420	(49,491)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	49,044
Balance at end of period (shares) at Oct. 31, 2019		16,921,547
Balance at end of period at Oct. 31, 2019	674,908	$ 357,304	366,464	(48,860)
Balance at beginning of period (shares) at Jul. 31, 2019		16,915,670
Balance at beginning of period at Jul. 31, 2019	651,168	$ 356,043	344,301	(49,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,163		22,163
Other comprehensive loss, net of tax	316			316
Stock-based compensation	1,178	$ 1,178
Exercise of stock-based compensation awards, net of amounts withheld for taxes (shares)		5,877
Stock repurchases	0
Balance at end of period (shares) at Oct. 31, 2019		16,921,547
Balance at end of period at Oct. 31, 2019	$ 674,908	$ 357,304	366,464	(48,860)
Balance at beginning of period (shares) at Apr. 30, 2020	16,926,537	16,926,537
Balance at beginning of period at Apr. 30, 2020	$ 700,538	$ 359,430	392,281	(51,173)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,485		16,485
Other comprehensive loss, net of tax	327			327
Stock-based compensation	961	$ 961
Exercise of stock-based compensation awards, net of amounts withheld for taxes (shares)		16,212
Exercise of stock-based compensation awards, net of amounts withheld for taxes	(534)	$ (534)
Employee benefit plan contributions (shares)		45,591
Employee benefit plan contributions	3,743	$ 3,743
Balance at end of period (shares) at Jul. 31, 2020		16,988,340
Balance at end of period at Jul. 31, 2020	$ 721,520	$ 363,600	408,766	(50,846)
Balance at beginning of period (shares) at Apr. 30, 2020	16,926,537	16,926,537
Balance at beginning of period at Apr. 30, 2020	$ 700,538	$ 359,430	392,281	(51,173)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 38,741
Stock repurchases (shares)	0
Balance at end of period (shares) at Oct. 31, 2020	16,993,260	16,993,260
Balance at end of period at Oct. 31, 2020	$ 745,194	$ 364,689	431,022	(50,517)
Balance at beginning of period (shares) at Jul. 31, 2020		16,988,340
Balance at beginning of period at Jul. 31, 2020	721,520	$ 363,600	408,766	(50,846)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,256		22,256
Other comprehensive loss, net of tax	329			329
Stock-based compensation	1,266	$ 1,266
Exercise of stock-based compensation awards, net of amounts withheld for taxes (shares)		4,920
Exercise of stock-based compensation awards, net of amounts withheld for taxes	$ (177)	$ (177)
Balance at end of period (shares) at Oct. 31, 2020	16,993,260	16,993,260
Balance at end of period at Oct. 31, 2020	$ 745,194	$ 364,689	$ 431,022	$ (50,517)