Property, Plant and Equipment (Details) - USD ($) $ in Thousands	9 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Apr. 30, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 459,900		$ 444,634
Less accumulated amortization and depreciation	(247,923)		(226,532)
Property, Plant and Equipment, Net, Total	211,977		218,102
Amortization and depreciation expense on property, plant and equipment	9,000	$ 5,100
Accumulated amortization on capital leases	30,400		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	113,892		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	287,065		274,723
Machinery and equipment - capital leases
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,391		30,270
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 12,599		$ 10,931