Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended			176 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015	Apr. 30, 2017
Stock Based Compensation And Stockholders Equity [Line Items]
Incentive and nonqualified options exercisable	1,365,586			1,365,586
Weighted-average grant date fair value of stock options granted	$ 1.96	$ 1.60	$ 2.04
Stock options exercised	754,811	306,848	490,912
Total intrinsic value of options exercised	$ 2,280,650	$ 1,219,406	$ 2,100,295
Fair value of grants vested	1,550,190	1,495,065	1,625,742
Unrecognized compensation cost related to unvested stock option	$ 2,400,000			$ 2,400,000
Weighted average period for Unrecognized compensation cost	1 year 6 months
Cost of common stock repurchased		$ 181,000	$ 1,100,000
Director | Initial Allocation
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant	5,000			5,000
Director | Quarterly Option Amount
Stock Based Compensation And Stockholders Equity [Line Items]
Options available for grant	3,000			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 available for grant	1,577,543			1,577,543
Options exercisable period, maximum	6 years
Shares options authorized for issuance	6,000,000	5,000,000		6,000,000
2011 Option Plan | Scenario, Plan
Stock Based Compensation And Stockholders Equity [Line Items]
Shares options authorized for issuance	7,000,000	6,000,000		7,000,000
2011 Option Plan | Non Qualified
Stock Based Compensation And Stockholders Equity [Line Items]
Incentive and nonqualified options exercisable	1,365,586	1,468,257	1,203,369	1,365,586
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