Stock-Based Compensation - Additional information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option for common stock			632,500	648,000
Stock-based compensation expense	$ 382,000	$ 342,000	$ 1,081,000	$ 1,117,000
Income tax benefit	125,000	109,000	353,000	325,000
Excess tax benefits from stock based compensation			349,000	108,000
Stock option exercised			549,804	294,006
Intrinsic value of options exercised			2,000,000	768,000
Unrecognized compensation cost related to unvested stock option	$ 3,900,000		$ 3,900,000
Weighted average period for Unrecognized compensation cost			1 year 9 months 18 days