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Consolidated Balance Sheets (Parenthetical)
$ in Thousands
Jun. 30, 2016
USD ($)
Vote
$ / shares
shares
Dec. 31, 2015
USD ($)
Vote
$ / shares
shares
Accounts receivable - trade, allowance for doubtful accounts | $ $ 1,070 $ 988
Property, plant and equipment, accumulated depreciation and amortization | $ $ 812,737 $ 789,766
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 40,500,000 40,064,000
Common stock, outstanding shares 40,500,000 40,064,000
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