Property, Plant And Equipment (Details) (USD $)	12 Months Ended
	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 271,089,000	$ 265,176,000
Less accumulated amortization and depreciation	(197,040,000)	(191,112,000)
Property, Plant and Equipment, Net, Total	74,049,000	74,064,000
Amortization and depreciation expense on property, plant and equipment	9,500,000	9,200,000	16,800,000
Accumulated amortization on capital leases	27,500,000	26,600,000
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,929,000	5,929,000
Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	68,224,000	67,444,000
Building And Improvements - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,202,000	11,202,000
Machinery And Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	155,162,000	152,154,000
Machinery And Equipment - Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28,111,000	26,966,000
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,461,000	$ 1,481,000