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Stock Options and Share-Based Compensation - Additional Information (Detail) - USD ($)
12 Months Ended
Aug. 26, 2015
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Aug. 25, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period   4 years      
Stock based compensation expense   $ 199,000 $ 192,000 $ 196,000  
Deferred income tax benefit   74,000 $ 72,000 $ 75,000  
Compensation expense not yet recorded   381,000      
Deferred income tax on remaining compensation expense   $ 141,000      
2010 Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares authorized under stock option plan   100,000      
Number of shares each director can purchase   10,000      
Terms of award   5 years      
Stock options plan, description   Options will be exercisable in four equal installments, one-fourth becoming exercisable on the next August 1 following the date of grant, and one-fourth becoming exercisable on August 1 of each of the next three years      
Shares available for future grants   25,000      
2008 Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares authorized under stock option plan         300,000
Shares available for future grants   255,100      
Increase in number of shares available under stock option plan 300,000        
2008 Plan [Member] | Maximum [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Terms of award   10 years