Other Assets - Other Noncurrent Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of other noncurrent assets
Spare parts inventory	$ 2,764	$ 3,532
Catalysts	2,962	4,479
Deposits	381	2,392
Other	933	2,227
Total	$ 7,040	$ 12,630