Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Apr. 30, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 467,818		$ 462,943
Less accumulated amortization and depreciation	(262,858)		(254,680)
Property, Plant and Equipment, Net, Total	204,960		208,263
Amortization and depreciation expense on property, plant and equipment	9,100	$ 8,800
Accumulated amortization on capital leases	31,000		30,800
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,751		4,751
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	114,529		114,421
Buildings and improvements - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,202		11,202
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	297,949		294,993
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,620		30,574
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 8,767		$ 7,002