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Stock Options and Share-Based Compensations - Summary of Stock Option Activity and Weighted Average Exercise Price (Detail) - $ / shares
 12 Months Ended
Apr. 30, 2016
Apr. 30, 2015
Apr. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Number of Shares, Outstanding at beginning of year 239,575 228,250 295,550
Number of Shares, Granted 40,200 45,800 46,600
Number of Shares, Canceled (10,400) (5,400) (16,650)
Number of Shares, Exercised (84,000) (29,075) (97,250)
Number of Shares, Outstanding at end of year 185,375 239,575 228,250
Number of Shares, Exercisable at end of year 92,075 136,700 113,175
Weighted Average Exercise Price, Outstanding at beginning of year $ 13.24 $ 12.17 $ 11.84
Weighted Average Exercise Price, Granted 16.92 17.78 15.26
Weighted Average Exercise Price, Canceled 16.37 11.74 10.10
Weighted Average Exercise Price, Exercised 11.44 12.28 13.00
Weighted Average Exercise Price, Outstanding at end of year 14.68 13.24 12.17
Weighted Average Exercise Price, Exercisable at end of year $ 12.83 $ 11.66 $ 11.90