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Summary of Significant Accounting Policies - Schedule of Property Plant and Equipment (Detail) - USD ($)
$ in Thousands
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Property, Plant and Equipment [Line Items]    
Property plant and equipment gross $ 51,568 $ 49,928
Less accumulated depreciation (37,541) (35,810)
Net property, plant and equipment 14,027 14,118
Land [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment gross 41 41
Building and Improvements [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment gross $ 15,892 $ 15,638
Building and Improvements [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Useful Life 10 years 10 years
Building and Improvements [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Useful Life 40 years 40 years
Machinery and Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property plant and equipment gross $ 35,635 $ 34,249
Machinery and Equipment [Member] | Minimum [Member]    
Property, Plant and Equipment [Line Items]    
Useful Life 5 years 5 years
Machinery and Equipment [Member] | Maximum [Member]    
Property, Plant and Equipment [Line Items]    
Useful Life 10 years 10 years