General (Policies)	9 Months Ended
General (Policies)	Sep. 30, 2018
Accounting Policies [Abstract]
Recently issued accounting standards	In February 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2016-02, "Leases (Topic 842)". The objective of ASU 2016-02 is to provide enhanced transparency and comparability among organizations by, among other things, recognizing right-of-use assets and lease liabilities on the balance sheet for all leases other than those that meet the definition of short-term leases and disclosing qualitative and quantitative information about lease arrangements. The new standard continues to classify leases as either finance or operating, with classification affecting the pattern of expense recognition in the statement of operations. ASU 2016-02 is effective for Century beginning January 1, 2019. Through September 30, 2018, we have determined the scope of arrangements impacted by the standard, and are evaluating the impact of adopting this standard to such arrangements. We are also in the process of determining the applicable key assumptions in applying the standard. In conjunction with the aforementioned implementation activities, we continue enhancing our internal processes and controls to capture arrangements that are likely to be in scope of the standard upon and post adoption. We anticipate the adoption of ASU 2016-02 to result in the recognition of right-of-use assets and lease liabilities for most leases currently accounted for as operating leases. We do not anticipate the adoption of this standard to have any impact to our cash flows. We continue to monitor modifications, clarifications or interpretations undertaken by the FASB and Securities and Exchange Commission ("SEC") and changes in our business and new arrangements, which may impact our current conclusions. We expect to adopt the standard on a modified retrospective basis.
Revenue recognition	We enter into contracts to sell primary aluminum to our customers. Revenue is recognized when our contractual obligations with our customer are satisfied. Our obligations under the contracts are satisfied when we transfer control of our primary aluminum to our customer which is generally upon shipment or delivery to customer-directed locations. The amount of consideration we receive, thus the revenue we recognize, is a function of volume delivered, market price of primary aluminum, as determined on the LME, plus regional delivery premiums and any value-added product premiums. The payment terms and conditions in our contracts vary and are not significant to our revenue. We complete an appropriate credit evaluation for each customer at contract inception. Customer payments are due in arrears and are recognized as accounts receivable - net and due from affiliates in our consolidated balance sheets. Accounts receivable - net increased $48.3 million from December 31, 2017 to September 30, 2018, driven primarily by the increase in direct sales to end users with longer payment terms than our related party customer. When the customer-directed location is our plant site, revenue is recognized only when the customer has specifically requested delivery to such location and control over the goods transfers upon delivery to such location. The goods must be complete, ready for physical transfer, separated from other inventory and we retain no further obligations.
Commitments and contingencies	It is our policy to accrue for costs associated with environmental assessments and remedial efforts when it becomes probable that a liability has been incurred and the costs can be reasonably estimated. All accrued amounts have been recorded without giving effect to any possible future recoveries. Costs for ongoing environmental compliance, including maintenance and monitoring are expensed as incurred.