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Stock Options and Share-Based Compensation - Fair Value of Options Granted Using Black-Scholes Model (Detail) - $ / shares
12 Months Ended
Aug. 30, 2017
Apr. 30, 2017
Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted   45,200 40,200
2008 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options granted 0 45,200 40,200
Weighted average expected stock price volatility   29.17% 30.45%
Expected option life   6 years 3 months 6 years 3 months
Average risk-free interest rate   2.10% 1.97%
Average dividend yield   3.00% 2.78%
Estimated fair value of each option   $ 5.04 $ 3.97