Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation — The consolidated financial statements include the accounts of Century Aluminum Company and our subsidiaries, after elimination of all intercompany transactions and accounts.
Revenue recognition	Revenue recognition — See Note 3. Revenue. We enter into contracts to sell mainly primary aluminum to our customers. Revenue is recognized when our performance obligations with our customers are satisfied. Our obligations under the contracts are satisfied when we transfer control of our primary aluminum to our customers 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, which is based on the LME, plus regional 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. In connection with our sales agreement with Glencore, we invoice Glencore prior to physical shipment of goods for a majority of production generated from each of our U.S. domestic smelters. For those sales, revenue is recognized only when Glencore has specifically requested such treatment and has made a commitment to purchase the product. The goods must be complete, ready for shipment and separated from other inventory with control over the goods passing to Glencore. We must retain no further performance obligations.
Cash and Cash Equivalents	Cash and Cash Equivalents — Cash and cash equivalents are comprised of cash, money market funds and short-term investments having original maturities of three months or less. The carrying amount of cash equivalents approximates fair value.
Accounts Receivable and Due from Affiliates	Accounts Receivable and Due from Affiliates — These amounts are net of an allowance for expected losses of $1.0 million at December 31, 2020 and 2019.
Inventories	Inventories — Our inventories are stated at the lower of cost or net realizable value, using the first-in, first-out ("FIFO") and the weighted average cost method. Due to the nature of our business, our inventory values are subject to market price changes and these changes can have a significant impact on cost of goods sold and gross profit in any period. Reductions in net realizable value below cost basis at the end of a period will have an impact on our cost of goods sold as this inventory is sold in subsequent periods.
Property, Plant and Equipment	Property, Plant and Equipment — Property, plant and equipment is stated at cost. Additions and improvements are capitalized when each asset is placed into service. Asset and accumulated depreciation accounts are relieved for dispositions with resulting gains or losses included in Other income (expense) – net. Maintenance and repairs are expensed as incurred. Depreciation of plant and equipment is provided for by the straight-line method over the following estimated useful lives: Building and improvements 10 to 45 years Machinery and equipment 5 to 35 years Technology and software 3 to 7 years
Impairment of long-lived assets	Impairment of long-lived assets — We evaluate our property, plant and equipment for potential impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable. If deemed unrecoverable, an impairment loss would be recognized for the amount by which the carrying amount exceeds the fair value of the assets. Impairment evaluation and fair value is based on estimates and assumptions that take into account our business plans and a long-term investment horizon.
Leases	Leases — We determine whether an arrangement is a lease at the inception of the arrangement based on the terms and conditions in the contract. A contract contains a lease if there is an identified asset which we have the right to control. We have made a policy election not to separate lease and non-lease components within contracts. We have also elected not to recognize the impact of short term leases in the right of use asset ("ROUA") and right of use liability ("ROUL") balances. Short term leases are leases that have a lease term less than one year and do not include a purchase option.
Income Taxes	Income Taxes — We account for income taxes using the asset and liability method, whereby deferred income taxes reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. In evaluating our ability to realize deferred tax assets, we use judgment to determine if it is more likely than not that some portion or all of a deferred tax asset will not be realized, and if a corresponding valuation allowance is required.
Defined Benefit Pension and Other Postretirement Benefits	Defined Benefit Pension and Other Postretirement Benefits — We sponsor defined benefit pension and OPEB plans for certain of our domestic hourly and salaried employees and a supplemental executive retirement benefit plan for certain current and former executive officers. Plan assets and obligations are measured annually or more frequently if there is a re-measurement event, based on the Company’s measurement date utilizing various actuarial assumptions. We attribute the service costs for the plans over the working lives of plan participants. The effects of actual results differing from our assumptions and the effects of changing assumptions are considered actuarial gains or losses. Actuarial gains or losses are recorded in Accumulated Other Comprehensive Income (Loss). We contribute to our defined benefit pension plans based upon actuarial and economic assumptions designed to achieve adequate funding of the projected benefit obligations and to meet the minimum funding requirements.
Postemployment Benefits	Postemployment Benefits — We provide certain postemployment benefits to certain former and inactive employees and their dependents during the period following employment, but before retirement. These benefits include salary continuance, supplemental unemployment and disability health care. We recognize the estimated future cost of providing postemployment benefits on an accrual basis over the active service life of the employee.
Derivative and Hedging	Derivatives and Hedging — As a global producer of primary aluminum, our operating results and cash flows from operations are subject to risk of fluctuations in the market prices of primary aluminum. We may from time to time enter into financial contracts to manage our exposure to such risk. Derivative instruments may consist of variable to fixed financial contracts and back-to-back fixed to floating arrangements for a portion of our sale of primary aluminum, where we receive fixed and pay floating prices from our customers and to counterparties, respectively. From time to time, we may manage our exposure to fluctuations in the market price of power through financial instruments designed to protect our downside risk exposure. We are also exposed to foreign currency risk, and we may manage our exposure by entering into foreign currency forward contracts or option contracts for forecasted transactions and projected cash flows for foreign currencies in future periods. Our derivatives are not designated as cash flow hedges. Derivative and hedging instruments are recorded in due from affiliates, derivative assets, other assets, due to affiliates, accrued and other current liabilities and other long term liabilities in the consolidated balance sheets at fair value. We value our derivative and hedging instruments using quoted market prices and other significant unobservable inputs. We recognize changes in fair value and settlements of derivative instruments in net gain (loss) on forward and derivative contracts in the consolidated statements of operations as they occur.
Foreign Currency	Foreign Currency – We are exposed to foreign currency risk due to fluctuations in the value of the U.S. dollar as compared to the Euro and the Icelandic krona ("ISK"). Grundartangi and Vlissingen use the U.S. dollar as their functional currency, as contracts for sales of aluminum and purchases of alumina and power are denominated in U.S. dollars. Transactions denominated in currencies other than the functional currency are recorded based on exchange rates at the time such transactions arise and any transaction gains and losses are reflected in Other income (expense) – net in the consolidated statements of operations.
Financial Instruments	Financial Instruments — Receivables, certain life insurance policies, payables, borrowings under revolving credit facilities and debt related to industrial revenue bonds ("IRBs") are carried at amounts that approximate fair value.
Earnings per share	Earnings per share — Basic earnings (loss) per share ("EPS") amounts are calculated by dividing earnings (loss) available to common stockholders by the weighted average number of common shares outstanding. Diluted EPS amounts assume the issuance of common stock for all potentially dilutive common shares outstanding. Because our capital structure consists of common stock and participating convertible preferred stock, we use the two-class method to calculate basic EPS, and incorporate the use of such method to determine diluted EPS. Our Series A Convertible Preferred Stock is a non-cumulative perpetual participating convertible preferred stock with no set dividend preferences. In periods where we report net losses, we do not allocate these losses to the convertible preferred stock for the computation of basic or diluted EPS.
Asset Retirement Obligations	Asset Retirement Obligations — We are subject to environmental regulations which create certain legal obligations related to the normal operations of our domestic primary aluminum smelter operations. Our asset retirement obligations ("AROs") consist primarily of costs associated with the disposal of spent potliner used in the reduction cells of our domestic facilities. AROs are recorded on a discounted basis at the time the obligation is incurred (when the potliner is put in service) and accreted over time for the change in the present value of the liability. We capitalize the asset retirement costs by increasing the carrying amount of the related long-lived assets and depreciating these assets over their remaining useful lives. Certain conditional asset retirement obligations ("CAROs") relate to the remediation of our primary aluminum facilities for hazardous material, such as landfill materials and asbestos which have not been recorded because they have an indeterminate settlement date. CAROs are a legal obligation to perform an asset retirement activity in which the timing and (or) method of settlement are conditional on a future event that may or may not be within our control.
Concentration of Credit Risk	Concentrations of Credit Risk — Financial instruments, which potentially expose us to concentrations of credit risk, consist principally of trade receivables. Our limited customer base increases our concentrations of credit risk with respect to trade receivables. We routinely assess the financial strength of our customers and collectability of our trade receivables and recognize an allowance based on our estimate of lifetime expected credit losses in accordance with the current expected credit loss ("CECL") model.
Share-Based Compensation	Share-Based Compensation — We measure the cost of employee services received in exchange for an award of equity instruments based on the fair value of the award on the grant date. We recognize the cost over the period during which an employee is required to provide service in exchange for the award. We issue shares to satisfy the requirements of our share-based compensation plans. At this time, we do not plan to issue treasury shares to support our share-based compensation plans, but we may in the future. We award performance units to certain officers and employees. The performance units may be settled in cash or common stock at the discretion of the Board. We have not issued any stock options since 2009.
Recently Issued Accounting Standards	Recently Issued Accounting Standards In December 2019, the FASB issued ASU 2019-12, “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes." Amendments include the removal of certain exceptions to the general principles of ASC 740 and to improve and simplify accounting for income taxes by clarifying and amending existing guidance. The ASU is effective for fiscal years beginning after December 15, 2020, including interim periods therein and early adoption is permitted. We will apply ASU 2019-12 beginning January 2021 and at this time, we do not expect the adoption to have a material effect on the consolidated financial statements. In March 2020, the Securities and Exchange Commission (“SEC”) issued a final rule that amends the disclosure requirements related issuers and guarantors of certain registered securities under SEC Regulation S-X Rule 3-10. The final rule allows registrants to provide alternative financial disclosures in either the registrant’s MD&A or financial statements, rather than the previous requirement under Rule 3-10, which required condensed consolidating financial information within the financial statements. It also simplifies the requirements in Rule 3-10 that currently must be met for a parent company to qualify for exceptions allowing it to provide alternative disclosures rather than full audited financial statements. The ruling also reduces the periods for which summarized financial information is required to be presented to the most recent (1) annual period and (2) year-to-date interim period. The final rule applies to annual reports on Form 10-K for fiscal years ending after January 4, 2021 and quarterly reports on Form 10-Q for quarterly periods ending after January 4, 2021 and registrants may voluntarily comply with the final rule before the effective date. The Company does not expect the future adoption, which is limited to disclosures only, to have a material effect on the Company’s consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

Basis of Presentation — The consolidated financial statements include the accounts of Century Aluminum Company and our subsidiaries, after elimination of all intercompany transactions and accounts.

Revenue recognition

Revenue recognition — See Note 3. Revenue.

We enter into contracts to sell mainly primary aluminum to our customers. Revenue is recognized when our performance obligations with our customers are satisfied. Our obligations under the contracts are satisfied when we transfer control of our primary aluminum to our customers 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, which is based on the LME, plus regional 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.

In connection with our sales agreement with Glencore, we invoice Glencore prior to physical shipment of goods for a majority of production generated from each of our U.S. domestic smelters. For those sales, revenue is recognized only when Glencore has specifically requested such treatment and has made a commitment to purchase the product. The goods must be complete, ready for shipment and separated from other inventory with control over the goods passing to Glencore. We must retain no further performance obligations.

Cash and Cash Equivalents

Cash and Cash Equivalents — Cash and cash equivalents are comprised of cash, money market funds and short-term investments having original maturities of three months or less. The carrying amount of cash equivalents approximates fair value.

Accounts Receivable and Due from Affiliates

Accounts Receivable and Due from Affiliates — These amounts are net of an allowance for expected losses of $1.0 million at December 31, 2020 and 2019.

Inventories

Inventories — Our inventories are stated at the lower of cost or net realizable value, using the first-in, first-out ("FIFO") and the weighted average cost method. Due to the nature of our business, our inventory values are subject to market price changes and these changes can have a significant impact on cost of goods sold and gross profit in any period. Reductions in net realizable value below cost basis at the end of a period will have an impact on our cost of goods sold as this inventory is sold in subsequent periods.

Property, Plant and Equipment

Property, Plant and Equipment — Property, plant and equipment is stated at cost. Additions and improvements are capitalized when each asset is placed into service. Asset and accumulated depreciation accounts are relieved for dispositions with resulting gains or losses included in Other income (expense) – net. Maintenance and repairs are expensed as incurred. Depreciation of plant and equipment is provided for by the straight-line method over the following estimated useful lives:

Building and improvements 10 to 45 years

Machinery and equipment 5 to 35 years

Technology and software 3 to 7 years

Impairment of long-lived assets

Impairment of long-lived assets — We evaluate our property, plant and equipment for potential impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable. If deemed unrecoverable, an impairment loss would be recognized for the amount by which the carrying amount exceeds the fair value of the assets. Impairment evaluation and fair value is based on estimates and assumptions that take into account our business plans and a long-term investment horizon.

Leases

Leases — We determine whether an arrangement is a lease at the inception of the arrangement based on the terms and conditions in the contract. A contract contains a lease if there is an identified asset which we have the right to control. We have made a policy election not to separate lease and non-lease components within contracts. We have also elected not to recognize the impact of short term leases in the right of use asset ("ROUA") and right of use liability ("ROUL") balances. Short term leases are leases that have a lease term less than one year and do not include a purchase option.

Income Taxes

Income Taxes — We account for income taxes using the asset and liability method, whereby deferred income taxes reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. In evaluating our ability to realize deferred tax assets, we use judgment to determine if it is more likely than not that some portion or all of a deferred tax asset will not be realized, and if a corresponding valuation allowance is required.

Defined Benefit Pension and Other Postretirement Benefits

Defined Benefit Pension and Other Postretirement Benefits — We sponsor defined benefit pension and OPEB plans for certain of our domestic hourly and salaried employees and a supplemental executive retirement benefit plan for certain current and former executive officers. Plan assets and obligations are measured annually or more frequently if there is a re-measurement event, based on the Company’s measurement date utilizing various actuarial assumptions. We attribute the service costs for the plans over the working lives of plan participants. The effects of actual results differing from our assumptions and the effects of changing assumptions are considered actuarial gains or losses. Actuarial gains or losses are recorded in Accumulated Other Comprehensive Income (Loss).

We contribute to our defined benefit pension plans based upon actuarial and economic assumptions designed to achieve adequate funding of the projected benefit obligations and to meet the minimum funding requirements.

Postemployment Benefits

Postemployment Benefits — We provide certain postemployment benefits to certain former and inactive employees and their dependents during the period following employment, but before retirement. These benefits include salary continuance, supplemental unemployment and disability health care. We recognize the estimated future cost of providing postemployment benefits on an accrual basis over the active service life of the employee.

Derivative and Hedging

Derivatives and Hedging — As a global producer of primary aluminum, our operating results and cash flows from operations are subject to risk of fluctuations in the market prices of primary aluminum. We may from time to time enter into financial contracts to manage our exposure to such risk. Derivative instruments may consist of variable to fixed financial contracts and back-to-back fixed to floating arrangements for a portion of our sale of primary aluminum, where we receive fixed and pay floating prices from our customers and to counterparties, respectively.

From time to time, we may manage our exposure to fluctuations in the market price of power through financial instruments designed to protect our downside risk exposure. We are also exposed to foreign currency risk, and we may manage our exposure by entering into foreign currency forward contracts or option contracts for forecasted transactions and projected cash flows for foreign currencies in future periods.

Our derivatives are not designated as cash flow hedges.

Derivative and hedging instruments are recorded in due from affiliates, derivative assets, other assets, due to affiliates, accrued and other current liabilities and other long term liabilities in the consolidated balance sheets at fair value. We value our derivative and hedging instruments using quoted market prices and other significant unobservable inputs. We recognize changes in fair value and settlements of derivative instruments in net gain (loss) on forward and derivative contracts in the consolidated statements of operations as they occur.

Foreign Currency

Foreign Currency – We are exposed to foreign currency risk due to fluctuations in the value of the U.S. dollar as compared to the Euro and the Icelandic krona ("ISK"). Grundartangi and Vlissingen use the U.S. dollar as their functional currency, as contracts for sales of aluminum and purchases of alumina and power are denominated in U.S. dollars. Transactions denominated in currencies other than the functional currency are recorded based on exchange rates at the time such transactions arise and any transaction gains and losses are reflected in Other income (expense) – net in the consolidated statements of operations.

Financial Instruments

Financial Instruments — Receivables, certain life insurance policies, payables, borrowings under revolving credit facilities and debt related to industrial revenue bonds ("IRBs") are carried at amounts that approximate fair value.

Earnings per share

Earnings per share — Basic earnings (loss) per share ("EPS") amounts are calculated by dividing earnings (loss) available to common stockholders by the weighted average number of common shares outstanding. Diluted EPS amounts assume the issuance of common stock for all potentially dilutive common shares outstanding. Because our capital structure consists of common stock and participating convertible preferred stock, we use the two-class method to calculate basic EPS, and incorporate the use of such method to determine diluted EPS.

Our Series A Convertible Preferred Stock is a non-cumulative perpetual participating convertible preferred stock with no set dividend preferences. In periods where we report net losses, we do not allocate these losses to the convertible preferred stock for the computation of basic or diluted EPS.

Asset Retirement Obligations

Asset Retirement Obligations — We are subject to environmental regulations which create certain legal obligations related to the normal operations of our domestic primary aluminum smelter operations. Our asset retirement obligations ("AROs") consist primarily of costs associated with the disposal of spent potliner used in the reduction cells of our domestic facilities. AROs are recorded on a discounted basis at the time the obligation is incurred (when the potliner is put in service) and accreted over time for the change in the present value of the liability. We capitalize the asset retirement costs by increasing the carrying amount of the related long-lived assets and depreciating these assets over their remaining useful lives.

Certain conditional asset retirement obligations ("CAROs") relate to the remediation of our primary aluminum facilities for hazardous material, such as landfill materials and asbestos which have not been recorded because they have an indeterminate settlement date. CAROs are a legal obligation to perform an asset retirement activity in which the timing and (or) method of settlement are conditional on a future event that may or may not be within our control.

Concentration of Credit Risk

Concentrations of Credit Risk — Financial instruments, which potentially expose us to concentrations of credit risk, consist principally of trade receivables. Our limited customer base increases our concentrations of credit risk with respect to trade receivables. We routinely assess the financial strength of our customers and collectability of our trade receivables and recognize an allowance based on our estimate of lifetime expected credit losses in accordance with the current expected credit loss ("CECL") model.

Share-Based Compensation

Share-Based Compensation — We measure the cost of employee services received in exchange for an award of equity instruments based on the fair value of the award on the grant date. We recognize the cost over the period during which an employee is required to provide service in exchange for the award. We issue shares to satisfy the requirements of our share-based compensation plans. At this time, we do not plan to issue treasury shares to support our share-based compensation plans, but we may in the future. We award performance units to certain officers and employees. The performance units may be settled in cash or common stock at the discretion of the Board. We have not issued any stock options since 2009.

Recently Issued Accounting Standards

In December 2019, the FASB issued ASU 2019-12, “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes." Amendments include the removal of certain exceptions to the general principles of ASC 740 and to improve and simplify accounting for income taxes by clarifying and amending existing guidance. The ASU is effective for fiscal years beginning after December 15, 2020, including interim periods therein and early adoption is permitted. We will apply ASU 2019-12 beginning January 2021 and at this time, we do not expect the adoption to have a material effect on the consolidated financial statements.

In March 2020, the Securities and Exchange Commission (“SEC”) issued a final rule that amends the disclosure requirements related issuers and guarantors of certain registered securities under SEC Regulation S-X Rule 3-10. The final rule allows registrants to provide alternative financial disclosures in either the registrant’s MD&A or financial statements, rather than the previous requirement under Rule 3-10, which required condensed consolidating financial information within the financial statements. It also simplifies the requirements in Rule 3-10 that currently must be met for a parent company to qualify for exceptions allowing it to provide alternative disclosures rather than full audited financial statements. The ruling also reduces the periods for which summarized financial information is required to be presented to the most recent (1) annual period and (2) year-to-date interim period. The final rule applies to annual reports on Form 10-K for fiscal years ending after January 4, 2021 and quarterly reports on Form 10-Q for quarterly periods ending after January 4, 2021 and registrants may voluntarily comply with the final rule before the effective date. The Company does not expect the future adoption, which is limited to disclosures only, to have a material effect on the Company’s consolidated financial statements.