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Inventories (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Inventory, Net [Abstract]    
Raw materials $ 54,914 $ 52,121
Work-in-process 28,302 34,025
Finished goods 18,821 15,988
Operating and other supplies 118,465 129,738
Total inventories $ 220,502 $ 231,872