Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 324,007	$ 308,405
Less accumulated amortization and depreciation	(216,074)	(209,073)
Property, Plant and Equipment, Net, Total	107,933	99,332
Amortization and depreciation expense on property, plant and equipment	14,200	11,600	$ 9,500
Accumulated amortization on capital leases	29,700	29,600
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	3,581	2,311
Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	81,172	80,042
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	187,836	182,937
Machinery And Equipment - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29,378	29,357
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 10,838	$ 2,556