Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2021	Jan. 31, 2020	Jan. 31, 2021	Jan. 31, 2020	Apr. 30, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 491,782		$ 491,782		$ 488,767
Less accumulated amortization and depreciation	(290,897)		(290,897)		(284,943)
Property, Plant and Equipment, Net, Total	200,885		200,885		203,824
Amortization and depreciation expense on property, plant and equipment	10,300	$ 9,300	32,500	$ 27,600
Accumulated amortization on capital leases	33,200		33,200		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	115,515		115,515		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	309,152		309,152		312,806
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	31,440		31,440		30,911
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 19,608		19,608		$ 8,164
Manufacturing Facility
Property, Plant and Equipment [Line Items]
Amortization and depreciation expense on property, plant and equipment			$ 1,300