Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2021	Oct. 31, 2020	Oct. 31, 2021	Oct. 31, 2020	Apr. 30, 2021
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 523,435		$ 523,435		$ 501,596
Less accumulated amortization and depreciation	(314,739)		(314,739)		(297,594)
Property, Plant and Equipment, Net, Total	208,696		208,696		204,002
Amortization and depreciation expense on property, plant and equipment	9,400	$ 10,600	19,100	$ 22,200
Finance Lease, Right-of-Use Asset, Accumulated Amortization	32,900		32,900		33,000
Humboldt Facility
Property, Plant and Equipment [Line Items]
Depreciation		$ 200		$ 1,300
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	117,566		117,566		116,103
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	326,474		326,474		315,371
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	31,232		31,232		31,386
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 32,096		$ 32,096		$ 22,669