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, 2018		17,503,922
Balance at beginning of period at Apr. 30, 2018	$ 581,665	$ 361,158	$ 269,576	$ (49,069)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	24,767		24,767
Other comprehensive loss, net of tax	307			307
Stock-based compensation	786	$ 786
Exercise of stock-based compensation awards, net of amounts withheld for taxes (shares)		43,048
Exercise of stock-based compensation awards, net of amounts withheld for taxes	(1,241)	$ (1,241)
Employee benefit plan contributions (shares)		41,408
Employee benefit plan contributions	3,623	$ 3,623
Balance at end of period (shares) at Jul. 31, 2018		17,588,378
Balance at end of period at Jul. 31, 2018	609,907	$ 364,326	294,343	(48,762)
Balance at beginning of period (shares) at Apr. 30, 2018		17,503,922
Balance at beginning of period at Apr. 30, 2018	581,665	$ 361,158	269,576	(49,069)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 43,255
Stock repurchases (shares)	(189,633)
Balance at end of period (shares) at Oct. 31, 2018		17,404,625
Balance at end of period at Oct. 31, 2018	$ 616,339	$ 361,560	303,234	(48,455)
Balance at beginning of period (shares) at Jul. 31, 2018		17,588,378
Balance at beginning of period at Jul. 31, 2018	609,907	$ 364,326	294,343	(48,762)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	18,488		18,488
Other comprehensive loss, net of tax	307			307
Stock-based compensation	$ 836	$ 836
Exercise of stock-based compensation awards, net of amounts withheld for taxes (shares)		5,880
Stock repurchases (shares)	(189,633)	(189,633)
Stock repurchases	$ (13,199)	$ (3,602)	(9,597)
Balance at end of period (shares) at Oct. 31, 2018		17,404,625
Balance at end of period at Oct. 31, 2018	$ 616,339	$ 361,560	303,234	(48,455)
Balance at beginning of period (shares) at Apr. 30, 2019	16,849,026	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	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
Stock repurchases (shares)	0
Balance at end of period (shares) at Oct. 31, 2019	16,921,547	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
Exercise of stock-based compensation awards, net of amounts withheld for taxes	$ 83	$ 83
Stock repurchases (shares)	0
Balance at end of period (shares) at Oct. 31, 2019	16,921,547	16,921,547
Balance at end of period at Oct. 31, 2019	$ 674,908	$ 357,304	$ 366,464	$ (48,860)