Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2020	Jul. 31, 2019	Apr. 30, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 490,853		$ 488,767
Less accumulated amortization and depreciation	(291,765)		(284,943)
Property, Plant and Equipment, Net, Total	199,088		203,824
Amortization and depreciation expense on property, plant and equipment	11,600	$ 9,100
Accumulated amortization on capital leases	32,800		32,300
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,431		4,431
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	120,995		120,819
Buildings and improvements - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,636		11,636
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	313,661		312,806
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,972		30,911
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	9,158		$ 8,164
Manufacturing Facility
Property, Plant and Equipment [Line Items]
Amortization and depreciation expense on property, plant and equipment	$ 1,100