Property, Plant And Equipment (Details) (USD $)	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 288,229,000	$ 281,880,000
Less accumulated amortization and depreciation	(214,165,000)	(206,505,000)
Property, plant and equipment, Total	74,064,000	75,375,000
Amortization and depreciation expense on property, plant and equipment	9,200,000	16,800,000	18,100,000
Accumulated amortization on capital leases	26,600,000	26,600,000
Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,929,000	5,929,000
Building And Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	65,245,000	65,750,000
Machinery And Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	177,393,000	169,406,000
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 Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	26,966,000	26,685,000
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,494,000	$ 2,908,000