Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2016	Apr. 30, 2015	Apr. 30, 2014
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 308,405	$ 287,856
Less accumulated amortization and depreciation	(209,073)	(202,340)
Property, Plant and Equipment, Net, Total	99,332	85,516
Amortization and depreciation expense on property, plant and equipment	11,600	9,500	$ 9,500
Accumulated amortization on capital leases	29,600	28,600
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,311	5,929
Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	80,042	69,412
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	182,937	159,680
Machinery And Equipment - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29,357	29,052
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,556	$ 12,581