Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Oct. 31, 2013	Apr. 30, 2013
Accounts receivable - trade, allowance for doubtful accounts	$ 2,054	$ 1,332
Property, plant and equipment, accumulated depreciation and amortization	$ 671,883	$ 645,567
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	39,011,000	38,662,000
Common stock, outstanding shares	39,011,000	38,662,000
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