Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Jul. 31, 2018	Jul. 31, 2017	Apr. 30, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 450,015		$ 444,634
Less accumulated amortization and depreciation	(233,715)		(226,532)
Property, Plant and Equipment, Net, Total	216,300		218,102
Amortization and depreciation expense on property, plant and equipment	8,800	$ 3,900
Accumulated amortization on capital leases	30,100		30,000
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,751		4,751
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	113,139		112,757
Buildings and improvements - capital leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,202		11,202
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	278,404		274,723
Machinery and equipment - capital leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,386		30,270
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 12,133		$ 10,931