Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2015	Apr. 30, 2014	Apr. 30, 2013
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 287,856	$ 271,089
Less accumulated amortization and depreciation	(202,340)	(197,040)
Property, Plant and Equipment, Net, Total	85,516	74,049
Amortization and depreciation expense on property, plant and equipment	9,500	9,500	$ 9,200
Accumulated amortization on capital leases	28,600	27,500
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,929	5,929
Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	69,412	68,224
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	159,680	155,162
Machinery And Equipment - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29,052	28,111
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 12,581	$ 2,461