Summary of Significant Accounting Policies - Schedule of Property Plant and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 54,648	$ 51,568
Less accumulated depreciation	(39,987)	(37,541)
Net property, plant and equipment	14,661	14,027
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	$ 16,489	$ 15,892
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	$ 38,118	$ 35,635
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