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Stock Options and Share-Based Compensation - Summary of Options Outstanding, Exercisable, and Weighted Average Exercise Prices within Exercise Price Range (Detail) - USD ($)
12 Months Ended
Apr. 30, 2016
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Options outstanding 185,375 239,575 228,250 295,550
Weighted average exercise price $ 14.68 $ 13.24 $ 12.17 $ 11.84
Exercise Price Range Range One [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise price range lower limit 8.59      
Exercise price range upper limit $ 12.66      
Options outstanding 64,275      
Weighted average exercise price $ 10.87      
Weighted average remaining contractual life 4 years 6 months 15 days      
Aggregate intrinsic value $ 376,000      
Options exercisable 56,925      
Weighted average exercise price $ 10.75      
Aggregate intrinsic value $ 340,000      
Exercise Price Range Range Two [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise price range lower limit $ 13.12      
Exercise price range upper limit $ 18.14      
Options outstanding 121,100      
Weighted average exercise price $ 16.70      
Weighted average remaining contractual life 7 years 5 months 5 days      
Aggregate intrinsic value $ 55,000      
Options exercisable 35,150      
Weighted average exercise price $ 16.20      
Aggregate intrinsic value $ 31,000