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Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Components of property, plant and equipment      
Land $ 23,566 $ 30,672 $ 28,472
Land improvements and leasehold improvements 71,236 79,631 77,849
Buildings and storage facilities 298,077 322,856 290,528
Machinery and equipment 382,321 392,418 374,107
Construction in progress 7,372 12,784 51,672
Property, plant and equipment, gross 782,572 838,361 822,628
Less: accumulated depreciation 359,530 388,309 375,361
Property, plant and equipment, net $ 423,042 $ 450,052 $ 447,267