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Inventories (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Inventory, Net [Abstract]    
Raw materials $ 106,210 $ 59,415
Work-in-process 49,584 35,539
Finished goods 40,852 26,613
Operating and other supplies 120,823 111,996
Inventories $ 317,469 $ 233,563