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Inventories (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Inventory, Net [Abstract]      
Raw materials $ 74,121 $ 59,415 $ 52,121
Work-in-process 41,331 35,539 34,025
Finished goods 22,440 26,613 15,988
Operating and other supplies 106,828 111,996 129,738
Inventories $ 244,720 $ 233,563 $ 231,872