CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jan. 31, 2012	Apr. 30, 2011
Accounts receivable - trade, allowance for doubtful accounts (in dollars)	$ 1,355	$ 920
Property, plant and equipment, accumulated depreciation and amortization (in dollars)	$ 667,776	$ 624,044
Class A Common Stock
Common stock, Authorized shares	100,000,000	100,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, issued shares	25,987,000	25,589,000
Common stock, outstanding shares	25,987,000	25,589,000
Class B Common Stock
Common stock, Authorized shares	1,000,000	1,000,000
Common stock, par value (in dollars per share)	$ 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