Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Inventory, Net [Abstract]
Raw materials	$ 64,377	$ 52,121
Work-in-process	30,848	34,025
Finished goods	19,817	15,988
Operating and other supplies	116,790	129,738
Total inventories	$ 231,832	$ 231,872