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Consolidated Balance Sheets (Parenthetical) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Consolidated Balance Sheets [Abstract]    
Accounts receivable, allowances $ 209 $ 217
Property, plant and equipment, accumulated depreciation $ 564,395 $ 514,585
Common stock, par value $ 0.001 $ 0.001
Common stock, shares authorized 75,000,000 75,000,000
Common stock, shares issued 46,760,696 46,410,405
Common stock, shares outstanding 41,434,804 41,084,463
Treasury stock, shares 5,325,892 5,325,942