Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 462,943	$ 444,634
Less accumulated amortization and depreciation	(254,680)	(226,532)
Property, Plant and Equipment, Net, Total	208,263	218,102
Amortization and depreciation expense on property, plant and equipment	36,200	21,900	$ 14,200
Accumulated amortization on capital leases	30,800	30,000
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,751	4,751
Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	114,421	112,757
Building And Improvements - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,202	11,202
Machinery And Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	294,993	274,723
Machinery And Equipment - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	30,574	30,270
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 7,002	$ 10,931