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Stock Options and Share-Based Compensation - Fair Value of Options Granted Using Black-Scholes Model (Detail) - $ / shares
12 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted 45,800 46,600 40,000
2008 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted 35,800 36,600 40,000
Weighted average expected stock price volatility 46.67% 50.58% 51.18%
Expected option life 6 years 3 months 6 years 3 months 6 years 3 months
Average risk-free interest rate 1.91% 1.80% 1.35%
Average dividend yield 2.61% 3.21% 4.34%
Estimated fair value of each option $ 6.60 $ 5.84 $ 3.86
2010 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted 10,000 10,000  
Weighted average expected stock price volatility 30.37% 36.98%  
Expected option life 3 years 4 months 17 days 3 years 3 months 29 days  
Average risk-free interest rate 1.28% 0.62%  
Average dividend yield 2.69% 3.33%  
Estimated fair value of each option $ 3.04 $ 2.73