Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Inventory, Net [Abstract]
Raw materials	$ 70,915	$ 59,415
Work-in-process	42,920	35,539
Finished goods	34,055	26,613
Operating and other supplies	113,866	111,996
Total inventories	$ 261,756	$ 233,563