Stock-Based Compensation - Additional information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option for common stock			313,000	343,000
Stock-based compensation expense	$ 361,000	$ 384,000	$ 700,000	$ 775,000
Income tax benefit	117,000	110,000	228,000	215,000
Excess tax benefits from stock based compensation			68,000	70,000
Stock option exercised			180,340	167,704
Intrinsic value of options exercised			450,000	472,000
Unrecognized compensation cost related to unvested stock option	$ 3,600,000		$ 3,600,000
Weighted average period for Unrecognized compensation cost			1 year 9 months 18 days