Consolidated Balance Sheets (Parenthetical) $ in Thousands	Jun. 30, 2015 USD ($) Vote $ / shares shares	Dec. 31, 2014 USD ($) Vote $ / shares shares
Accounts receivable - trade, allowance for doubtful accounts | $	$ 1,903	$ 2,153
Property, plant and equipment, accumulated depreciation and amortization | $	$ 762,442	$ 736,839
Class A Common Stock [Member]
Common stock, authorized shares	100,000,000	100,000,000
Common stock, par value | $ / shares	$ 0.01	$ 0.01
Common stock, issued shares	39,927,000	39,587,000
Common stock, outstanding shares	39,927,000	39,587,000
Class B Common Stock [Member]
Common stock, authorized shares	1,000,000	1,000,000
Common stock, par value | $ / shares	$ 0.01	$ 0.01
Common stock, votes (in votes per share) | Vote	10	10
Common stock, issued shares	988,000	988,000
Common stock, outstanding shares	988,000	988,000