Property, Plant and Equipment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jan. 31, 2018	Apr. 30, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 436,437	$ 324,007
Less accumulated amortization and depreciation	(225,809)	(216,074)
Property, Plant and Equipment, Net, Total	210,628	107,933
Amortization and depreciation expense on property, plant and equipment	12,900	10,500
Accumulated amortization on capital leases	30,000	29,700
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,751	3,581
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	93,902	81,172
Buildings and improvements - capital leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,203	11,202
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	268,063	187,836
Machinery and equipment - capital leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29,918	29,378
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 28,600	$ 10,838