Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2020	Oct. 31, 2019	Oct. 31, 2020	Oct. 31, 2019	Apr. 30, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 487,685		$ 487,685		$ 488,767
Less accumulated amortization and depreciation	(288,790)		(288,790)		(284,943)
Property, Plant and Equipment, Net, Total	198,895		198,895		203,824
Amortization and depreciation expense on property, plant and equipment	10,600	$ 9,200	22,200	$ 18,300
Accumulated amortization on capital leases	32,500		32,500		32,300
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,431		4,431		4,431
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	121,743		121,743		120,819
Buildings and improvements - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,636		11,636		11,636
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	305,692		305,692		312,806
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,870		30,870		30,911
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	13,313		13,313		$ 8,164
Manufacturing Facility
Property, Plant and Equipment [Line Items]
Amortization and depreciation expense on property, plant and equipment	$ 200		$ 1,300