Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019	Apr. 30, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 488,024		$ 488,024		$ 462,943
Less accumulated amortization and depreciation	(279,244)		(279,244)		(254,680)
Property, Plant and Equipment, Net, Total	208,780		208,780		208,263
Amortization and depreciation expense on property, plant and equipment	9,300	$ 9,000	27,600	$ 26,800
Accumulated amortization on capital leases	31,900		31,900		30,800
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,431		4,431		4,751
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	115,004		115,004		114,421
Buildings and improvements - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,676		11,676		11,202
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	308,141		308,141		294,993
Machinery and equipment - finance leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,772		30,772		30,574
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 18,000		$ 18,000		$ 7,002