Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended			188 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2018
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant	1,208,000
Incentive and nonqualified options exercisable	965,416			965,416
Weighted-average grant date fair value of stock options granted	$ 2.01	$ 1.96	$ 1.60
Stock options exercised	984,310	754,811	306,848
Total intrinsic value of options exercised	$ 2,999,372	$ 2,280,650	$ 1,219,406
Fair value of grants vested	1,287,222	$ 1,550,190	1,495,065
Unrecognized compensation cost related to unvested stock option	$ 3,300,000			$ 3,300,000
Weighted average period for Unrecognized compensation cost	1 year 9 months 18 days
Cost of common stock repurchased			$ 181,000
Director | Initial Allocation
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant	5,000
Director | Quarterly Option Amount
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant	3,000
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 exercisable period, maximum	6 years
Shares options authorized for issuance	7,000,000			7,000,000
Options available for grant	1,396,543			1,396,543
Number of additional shares authorized	1,000,000
2011 Option Plan | Non Qualified
Stock Based Compensation And Stockholders Equity [Line Items]
Incentive and nonqualified options exercisable	965,416	1,365,586	1,468,257	965,416
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%			10.00%
Common stock dividends declared	$ 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			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%			10.00%
Options exercisable period, maximum	5 years