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Inventories (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Inventory, Net [Abstract]    
Raw materials $ 80,134 $ 59,415
Work-in-process 43,931 35,539
Finished goods 23,169 26,613
Operating and other supplies 107,377 111,996
Total inventories $ 254,611 $ 233,563