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Benefit plans (Detail) - USD ($)
12 Months Ended
Apr. 30, 2017
May 01, 2016
May 03, 2015
Compensation and Retirement Disclosure [Abstract]      
Contributions to defined contribution plans $ 924,000 $ 843,000 $ 798,000
Contributions to nonqualified deferred compensation plan 185,000 180,000 $ 174,000
deferred compensation 5,520,000 4,686,000  
long-term investments - rabbi trust $ 5,466,000 $ 4,025,000