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Consolidated Balance Sheets (Parenthetical)
$ in Thousands
Dec. 31, 2015
USD ($)
Vote
$ / shares
shares
Dec. 31, 2014
USD ($)
Vote
$ / shares
shares
Apr. 30, 2014
USD ($)
Vote
$ / shares
shares
Accounts receivable - trade, allowance for doubtful accounts | $ $ 988 $ 2,153 $ 1,672
Property, plant and equipment, accumulated depreciation and amortization | $ $ 789,766 $ 736,839 $ 695,935
Class A Common Stock [Member]      
Common stock, authorized shares 100,000,000 100,000,000 100,000,000
Common stock, par value | $ / shares $ 0.01 $ 0.01 $ 0.01
Common stock, issued shares 40,064,000 39,587,000 39,086,000
Common stock, outstanding shares 40,064,000 39,587,000 39,086,000
Class B Common Stock [Member]      
Common stock, authorized shares 1,000,000 1,000,000 1,000,000
Common stock, par value | $ / shares $ 0.01 $ 0.01 $ 0.01
Common stock, votes (in votes per share) | Vote 10 10 10
Common stock, issued shares 988,000 988,000 988,000
Common stock, outstanding shares 988,000 988,000 988,000