Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2022	Jan. 31, 2021	Jan. 31, 2022	Jan. 31, 2021	Apr. 30, 2021
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 528,093		$ 528,093		$ 501,596
Less accumulated amortization and depreciation	(319,365)		(319,365)		(297,594)
Property, Plant and Equipment, Net, Total	208,728		208,728		204,002
Amortization and depreciation expense on property, plant and equipment	9,200	$ 10,300	28,300	$ 32,500
Finance Lease, Right-of-Use Asset, Accumulated Amortization	32,700		32,700		33,000
Humboldt Facility
Property, Plant and Equipment [Line Items]
Depreciation		$ 0		$ 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	118,848		118,848		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	325,219		325,219		315,371
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	31,094		31,094		31,386
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 36,865		$ 36,865		$ 22,669