Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2023
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 5. 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. We have also entered into agreements with our customers to sell certain amounts of alumina at market-based prices. 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 agreements with certain customers, including Glencore, we invoice the customer 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 the customer 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 the customer. 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 $0.5 million at both December 31, 2023 and 2022.
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 (loss) income - 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 The Company incurs deferred costs during the development stage of a mine life cycle. Such costs include the construction of access and haul roads, detailed drilling and geological analysis to further define the grade and quality of the known bauxite, and overburden removal costs. These costs relate to sections of the related mines where the Company is currently extracting bauxite or preparing for production in the near term. These sections are outlined and planned incrementally and generally are mined over periods outlined in the Company's mine plans. The amount of geological drilling and testing necessary to determine the economic viability of the bauxite deposit being mined is such that the reserves are considered to be proven. Mineral reserves are amortized on a units-of-production basis.
Impairment of Long-Lived Assets	Impairment of long-lived assets — The Company reviews property, plant and equipment ("long-lived assets") for impairment whenever events or changes in circumstances, known as triggering events, indicate that the carrying amount of a long-lived asset or an asset group may not be recoverable. Management considers various factors when determining if long-lived assets should be evaluated for impairment, including a significant adverse change in the business climate or industry conditions (such as sustained decreases in commodity prices, volatility in energy costs, and the global economy), a current period operating or cash flow loss combined with a history of losses, a significant adverse change in the extent or manner in which an asset is used or a current expectation that the asset will be sold or otherwise disposed of before the end of its useful life. If a triggering event is identified, the Company determines if the long-lived asset or asset group is recoverable. Recoverability is measured by comparison of the carrying amount of a long-lived asset or asset group held and used to estimate undiscounted future net cash flows expected to be generated by the long-lived asset or asset group. Impairment evaluation and fair value is based on estimates and assumptions that take into account our business plans and a long-term investment horizon, including consideration of commodity pricing, energy costs and other global economic conditions which may have an adverse effect on recoverability. If deemed unrecoverable, an impairment loss would be recognized for the amount by which the carrying amount exceeds the estimated fair value of the long-lived asset or asset group.
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, derivative liabilities and derivative liabilities - less current portion 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"), and the Chinese renminbi. Grundartangi, Vlissingen and Jamalco use the U.S. dollar as their functional currency, as contracts for sales of aluminum and alumina 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 (loss) income - 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 using the two-class method. Under the two-class method, net income is allocated between shares of common stock and other participating securities based on their participating rights. Net loss is not allocated to other participating securities if they are not obligated to share in the losses based on their contractual terms. Diluted earnings per share is calculated by dividing net income attributable to common shareholders by the weighted average number of common and dilutive common equivalent shares outstanding during the period. Common equivalent shares are not included in the denominator of the diluted loss per share calculation when inclusion of such shares would be anti-dilutive. The dilutive effect to earnings per share is determined using the "if converted" method whereby, if the conversion of the convertible notes would be dilutive, interest expense on the outstanding notes is added back to the diluted earnings numerator and all of the potentially dilutive shares are included in the diluted common shares outstanding denominator for the computation of diluted earnings per share. 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 bauxite mine and alumina refinery and our domestic primary aluminum smelter operations. Our asset retirement obligations ("AROs") consist primarily of costs associated with mine reclamation obligations, closure of bauxite residue areas, landfill closure, and the disposal of spent potliner used in the reduction cells of our domestic smelters. AROs are recorded on a discounted basis at the time the obligation is incurred (when the potliner is put in service or upon disturbance of lands to be mined) 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") related to the remediation of our primary aluminum facilities for hazardous material, such as landfill materials and asbestos, 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.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2023

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 5. 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. We have also entered into agreements with our customers to sell certain amounts of alumina at market-based prices.

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 agreements with certain customers, including Glencore, we invoice the customer 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 the customer 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 the customer. 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 $0.5 million at both December 31, 2023 and 2022.

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 (loss) income - 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

The Company incurs deferred costs during the development stage of a mine life cycle. Such costs include the construction of access and haul roads, detailed drilling and geological analysis to further define the grade and quality of the known bauxite, and overburden removal costs. These costs relate to sections of the related mines where the Company is currently extracting bauxite or preparing for production in the near term. These sections are outlined and planned incrementally and generally are mined over periods outlined in the Company's mine plans. The amount of geological drilling and testing necessary to determine the economic viability of the bauxite deposit being mined is such that the reserves are considered to be proven. Mineral reserves are amortized on a units-of-production basis.

Impairment of Long-Lived Assets

Impairment of long-lived assets — The Company reviews property, plant and equipment ("long-lived assets") for impairment whenever events or changes in circumstances, known as triggering events, indicate that the carrying amount of a long-lived asset or an asset group may not be recoverable. Management considers various factors when determining if long-lived assets should be evaluated for impairment, including a significant adverse change in the business climate or industry conditions (such as sustained decreases in commodity prices, volatility in energy costs, and the global economy), a current period operating or cash flow loss combined with a history of losses, a significant adverse change in the extent or manner in which an asset is used or a current expectation that the asset will be sold or otherwise disposed of before the end of its useful life. If a triggering event is identified, the Company determines if the long-lived asset or asset group is recoverable. Recoverability is measured by comparison of the carrying amount of a long-lived asset or asset group held and used to estimate undiscounted future net cash flows expected to be generated by the long-lived asset or asset group. Impairment evaluation and fair value is based on estimates and assumptions that take into account our business plans and a long-term investment horizon, including consideration of commodity pricing, energy costs and other global economic conditions which may have an adverse effect on recoverability. If deemed unrecoverable, an impairment loss would be recognized for the amount by which the carrying amount exceeds the estimated fair value of the long-lived asset or asset group.

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, derivative liabilities and derivative liabilities - less current portion 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"), and the Chinese renminbi. Grundartangi, Vlissingen and Jamalco use the U.S. dollar as their functional currency, as contracts for sales of aluminum and alumina 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 (loss) income - 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 using the two-class method. Under the two-class method, net income is allocated between shares of common stock and other participating securities based on their participating rights. Net loss is not allocated to other participating securities if they are not obligated to share in the losses based on their contractual terms. Diluted earnings per share is calculated by dividing net income attributable to common shareholders by the weighted average number of common and dilutive common equivalent shares outstanding during the period. Common equivalent shares are not included in the denominator of the diluted loss per share calculation when inclusion of such shares would be anti-dilutive.

The dilutive effect to earnings per share is determined using the "if converted" method whereby, if the conversion of the convertible notes would be dilutive, interest expense on the outstanding notes is added back to the diluted earnings numerator and all of the potentially dilutive shares are included in the diluted common shares outstanding denominator for the computation of diluted earnings per share.

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 bauxite mine and alumina refinery and our domestic primary aluminum smelter operations. Our asset retirement obligations ("AROs") consist primarily of costs associated with mine reclamation obligations, closure of bauxite residue areas, landfill closure, and the disposal of spent potliner used in the reduction cells of our domestic smelters. AROs are recorded on a discounted basis at the time the obligation is incurred (when the potliner is put in service or upon disturbance of lands to be mined) 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") related to the remediation of our primary aluminum facilities for hazardous material, such as landfill materials and asbestos, 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.