Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands		Total	Class A Common Shares	Class B Common Shares	Common Stock Class A Common Shares		Common Stock Class B Common Shares	Additional Paid-in Capital	Retained Earnings	Treasury Stock
Beginning balance (in shares) at Apr. 30, 2018					33,141,764		2,057,390
Beginning balance at Apr. 30, 2018		$ 112,584			$ 3,314		$ 205	$ 131,258	$ 3,366	$ (25,559)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from stock options exercised (in shares)		343,713			343,509	[1]
Proceeds from stock options exercised	[1]	$ 3,086			$ 35			3,051
Conversion of Class B shares into Class A shares (in shares)	[1]				235,803		(235,803)
Conversion of Class B shares into Class A shares	[1]				$ 23		$ (23)
Stock-based compensation		841						841
Net earnings		2,628							2,628
Dividends declared	[1]	(6,804)							(6,804)
Ending balance (in shares) at Oct. 31, 2018					33,721,076		1,821,587
Ending balance at Oct. 31, 2018		114,088			$ 3,372		$ 182	135,150	943	(25,559)
Beginning balance (in shares) at Jul. 31, 2018					33,673,376		1,821,587
Beginning balance at Jul. 31, 2018		115,387			$ 3,367		$ 182	134,292	3,105	(25,559)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from stock options exercised (in shares)	[1]				47,700
Proceeds from stock options exercised	[1]	420			$ 5			415
Stock-based compensation		443						443
Net earnings		1,243							1,243
Dividends declared		(3,405)							(3,405)
Ending balance (in shares) at Oct. 31, 2018					33,721,076		1,821,587
Ending balance at Oct. 31, 2018		114,088			$ 3,372		$ 182	135,150	943	(25,559)
Beginning balance (in shares) at Apr. 30, 2019			33,979,739	1,821,587	33,979,739		1,821,587
Beginning balance at Apr. 30, 2019		$ 114,607			$ 3,398		$ 182	138,315	(1,729)	(25,559)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from stock options exercised (in shares)		650,943			650,943
Proceeds from stock options exercised		$ 6,358			$ 65			6,293
Stock-based compensation		946						946
Net earnings		2,911							2,911
Dividends declared	[1]	(6,956)							(6,956)
Ending balance (in shares) at Oct. 31, 2019			34,630,682	1,821,587	34,630,682		1,821,587
Ending balance at Oct. 31, 2019		117,866			$ 3,463		$ 182	145,554	(5,774)	(25,559)
Beginning balance (in shares) at Jul. 31, 2019					34,131,239		1,821,587
Beginning balance at Jul. 31, 2019		114,203			$ 3,413		$ 182	140,195	(4,028)	(25,559)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from stock options exercised (in shares)					499,443
Proceeds from stock options exercised		4,906			$ 50			4,856
Stock-based compensation		503						503
Net earnings		1,759							1,759
Dividends declared		(3,505)							(3,505)
Ending balance (in shares) at Oct. 31, 2019			34,630,682	1,821,587	34,630,682		1,821,587
Ending balance at Oct. 31, 2019		$ 117,866			$ 3,463		$ 182	$ 145,554	$ (5,774)	$ (25,559)

[1] Amounts adjusted for rounding