CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jul. 31, 2012	Apr. 30, 2012
Accounts receivable - trade, allowance for doubtful accounts	$ 936	$ 740
Property, plant and equipment, accumulated depreciation and amortization	$ 608,041	$ 593,206
Class A Common Stock [Member]
Common Stock, Authorized shares	100,000,000	100,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, issued shares	26,367,000	25,991,000
Common Stock, outstanding shares	26,367,000	25,991,000
Class B Common Stock [Member]
Common Stock, Authorized shares	1,000,000	1,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, votes (in votes per share)	$ 10	$ 10
Common stock, issued shares	988,000	988,000
Common Stock, outstanding shares	988,000	988,000