Shareholders' Equity - Additional Information (Details) - USD ($)	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018	Aug. 19, 2002
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant (in shares)	1,113,000
Incentive and nonqualified options exercisable (in shares)	900,610
Weighted-average grant date fair value of stock options granted (in usd per share)	$ 3.20	$ 2.58	$ 2.01
Exercised (in shares)	1,020,910	602,176	984,310
Total intrinsic value of options exercised	$ 5,569,882	$ 2,441,830	$ 2,999,372
Fair value of grants vested	1,786,342	$ 1,470,840	$ 1,287,222
Unrecognized compensation cost related to unvested stock option	$ 3,400,000
Weighted average period for unrecognized compensation cost	1 year 10 months 24 days
2001 Option Plan
Stock Based Compensation And Stockholders Equity [Line Items]
Stock option plan effective date	Sep. 01, 2000
2011 Option Plan
Stock Based Compensation And Stockholders Equity [Line Items]
Stock option plan effective date	May 17, 2010
Options available for grant (in shares)	2,500,000
Options exercisable period, maximum	6 years
Options authorized for issuance (in shares)		2,500,000
Options available for grant (in shares)	1,529,143
2011 Option Plan | Non Qualified
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant (in shares)	10,000
Incentive and nonqualified options exercisable (in shares)	900,610	1,086,180	965,416
2011 Option Plan | Quarterly Option Amount
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant (in shares)	4,000
1997 Stock Plan
Stock Based Compensation And Stockholders Equity [Line Items]
Stock option plan effective date	Jul. 09, 2009
Class A Common Shares
Stock Based Compensation And Stockholders Equity [Line Items]
Percentage of board of directors entitled to elect	25.00%
Ownership compared to both classes of common stock	10.00%
Common stock dividends declared (in usd per share)	$ 0.05
Common stock shares repurchased	4,588,632
Cost of common stock repurchased	$ 25,600,000
Class A Common Shares | Shares Stock Repurchase Plan, August 19, 2002
Stock Based Compensation And Stockholders Equity [Line Items]
Approved number of shares to be repurchased				2,000,000
Common stock shares repurchased	1,053,679
Cost of common stock repurchased	$ 6,200,000
Shareholder Holding More Than 10% of Outstanding Shares | 2011 Option Plan
Stock Based Compensation And Stockholders Equity [Line Items]
Ownership compared to both classes of common stock	10.00%
Options exercisable period, maximum	5 years