CENTURY ALUMINUM CO, 10-K filed on 3/15/2024
Annual Report
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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Mar. 14, 2024
Jun. 30, 2023
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2023    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-34474    
Entity Registrant Name CENTURY ALUMINUM COMPANY    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 13-3070826    
Entity Address, Address Line One One South Wacker Drive    
Entity Address, Postal Zip Code 60606    
Entity Address, Address Line Two Suite 1000    
Entity Address, City or Town Chicago    
Entity Address, State or Province IL    
City Area Code 312    
Local Phone Number 696-3101    
Title of 12(b) Security Common Stock, $0.01 par value per share    
Trading Symbol CENX    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 453
Entity Common Stock, Shares Outstanding   92,695,933  
Documents Incorporated by Reference
All or a portion of Items 10 through 14 in Part III of this Form 10-K are incorporated by reference to the Registrant’s definitive proxy statement on Schedule 14A for its 2024 Annual Meeting of Stockholders, which will be filed within 120 days after the close of the fiscal year covered by this report on Form 10-K, or if the Registrant’s Schedule 14A is not filed within such period, will be included in an amendment to this Report on Form 10-K which will be filed within such 120 day period.
    
Entity Central Index Key 0000949157    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Amendment Flag false    
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Audit Information
 12 Months Ended
Dec. 31, 2023
Audit Information [Abstract]  
Auditor Name Deloitte & Touche LLP
Auditor Location Chicago, Illinois
Auditor Firm ID 34
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CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($)
shares in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
NET SALES:      
Total net sales $ 2,185.4 $ 2,777.3 $ 2,212.5
Cost of goods sold 2,093.5 2,730.6 2,088.3
Gross profit 91.9 46.7 124.2
Selling, general and administrative expenses 44.3 37.5 57.6
Asset impairment 0.0 159.4 0.0
Other operating expense - net 15.8 0.0 0.6
Operating income (loss) 31.8 (150.2) 66.0
Interest income 2.0 0.5 0.8
Net (loss) gain on forward and derivative contracts (61.8) 197.1 (212.4)
Loss on early extinguishment of debt 0.0 0.0 (24.7)
Other (loss) income - net (3.3) 15.3 3.1
(Loss) income before income taxes (66.8) 33.4 (197.6)
Income tax benefit (expense) 14.6 (47.4) 30.6
Loss before equity in earnings of joint ventures (52.2) (14.0) (167.0)
Equity in losses of joint ventures (0.1) (0.1) (0.1)
Net loss (52.3) (14.1) (167.1)
Net loss attributable to noncontrolling interests (9.2) 0.0 0.0
Net loss $ (43.1) $ (14.1) $ (167.1)
LOSS ATTRIBUTABLE TO CENTURY STOCKHOLDERS PER COMMON SHARE:      
Basic (in dollars per share) $ (0.47) $ (0.15) $ (1.85)
Diluted (in dollars per share) $ (0.47) $ (0.15) $ (1.85)
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING:      
Basic (in shares) 92,400 91,400 90,200
Diluted (in shares) 92,400 91,400 90,200
Related Party      
NET SALES:      
Total net sales $ 1,612.1 $ 1,671.1 $ 1,337.0
Interest expense (1.8) 0.0 (1.6)
Nonrelated Party      
NET SALES:      
Total net sales 573.3 1,106.2 875.5
Interest expense $ (33.7) $ (29.3) $ (28.8)
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CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Comprehensive loss:      
Net loss $ (52.3) $ (14.1) $ (167.1)
Other comprehensive (loss) income before income tax effect:      
Net loss on foreign currency cash flow hedges reclassified as income (0.1) (0.2) (0.1)
Defined benefit plans and other postretirement benefits:      
Net (gain) loss arising during the period (10.1) (5.9) 31.6
OPEB curtailment gain, net 0.0 (8.9) 0.0
Amortization of prior service benefit (cost) during the period 0.1 (1.2) (3.1)
Amortization of net loss during the period 6.2 4.8 8.4
Other comprehensive (loss) income before income tax effect (3.9) (11.4) 36.8
Income tax effect 0.0 (0.3) (0.3)
Other comprehensive (loss) income (3.9) (11.7) 36.5
Comprehensive loss (56.2) (25.8) (130.6)
Comprehensive loss attributable to noncontrolling interests (9.2) 0.0 0.0
Comprehensive loss attributable to Century stockholders $ (47.0) $ (25.8) $ (130.6)
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CONSOLIDATED BALANCE SHEETS - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
ASSETS    
Cash and cash equivalents $ 88.8 $ 54.3
Restricted cash 1.5 1.2
Accounts receivable - net 53.7 66.9
Non-trade receivables 36.2 0.0
Manufacturing credit receivable 59.3 0.0
Inventories 477.0 398.8
Derivative assets 2.9 127.3
Prepaid and other current assets 27.5 24.5
Total current assets 767.1 677.8
Property, plant and equipment - net 1,004.2 744.4
Other assets 75.2 49.8
TOTAL 1,846.5 1,472.0
LIABILITIES:    
Accounts payable, trade 249.5 167.3
Accrued compensation and benefits 38.1 33.0
Accrued and other current liabilities 50.9 37.6
Derivative liabilities 1.4 9.7
Deferred credit - preliminary bargain purchase gain 273.4 0.0
Total current liabilities 763.0 410.7
Long-term debt 430.9 381.6
Accrued benefits costs - less current portion 120.3 118.0
Other liabilities 66.3 31.4
Deferred taxes 72.4 103.1
Asset retirement obligations - less current portion 49.5 19.6
Total noncurrent liabilities 739.4 662.0
COMMITMENTS AND CONTINGENCIES (NOTE 17)
SHAREHOLDERS’ EQUITY:    
Preferred stock (Note 9) 0.0 0.0
Common stock (Note 9) 1.0 1.0
Additional paid-in capital 2,542.9 2,539.6
Treasury stock, at cost (86.3) (86.3)
Accumulated other comprehensive loss (97.9) (94.0)
Accumulated deficit (2,004.1) (1,961.0)
Total Century shareholders’ equity 355.6 399.3
Noncontrolling interest (11.5) 0.0
Total equity 344.1 399.3
TOTAL 1,846.5 1,472.0
Related Party    
ASSETS    
Due from affiliates 20.2 4.8
LIABILITIES:    
Due to affiliates 101.4 17.0
Current maturities of long-term debt 10.0 0.0
Due to affiliates - less current portion 0.0 8.3
Nonrelated Party    
LIABILITIES:    
Current maturities of long-term debt $ 38.3 $ 146.1
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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($)
$ in Millions
Total
Total Century equity
Preferred stock
Common stock
Additional paid-in capital
Treasury stock, at cost
Accumulated other comprehensive loss
Accumulated deficit
Noncontrolling interest
Beginning balance (in shares) at Dec. 31, 2020     63,589            
Beginning balance at Dec. 31, 2020 $ 546.1 $ 546.1 $ 0.0 $ 1.0 $ 2,530.0 $ (86.3) $ (118.8) $ (1,779.8) $ 0.0
Beginning balance (in shares) at Dec. 31, 2020       90,055,797          
Increase (Decrease) in Stockholders' Equity [Roll Forward]                  
Net loss (167.1) (167.1)           (167.1)  
Other comprehensive income (loss) 36.5 36.5         36.5    
Share-based compensation (in shares)       671,218          
Share-based compensation 10.1 10.1     10.1        
Conversion of preferred stock to common stock     (5,047) 504,596          
Capped call premiums (4.6) (4.6)     (4.6)        
Ending balance (in shares) at Dec. 31, 2021     58,542            
Ending balance at Dec. 31, 2021 421.0 421.0 $ 0.0 $ 1.0 2,535.5 (86.3) (82.3) (1,946.9) 0.0
Ending balance (in shares) at Dec. 31, 2021       91,231,611          
Increase (Decrease) in Stockholders' Equity [Roll Forward]                  
Net loss (14.1) (14.1)           (14.1)  
Other comprehensive income (loss) (11.7) (11.7)         (11.7)    
Share-based compensation (in shares)       623,582          
Share-based compensation 4.1 4.1     4.1        
Conversion of preferred stock to common stock     (4,688) 468,785          
Ending balance (in shares) at Dec. 31, 2022     53,854            
Ending balance at Dec. 31, 2022 $ 399.3 399.3 $ 0.0 $ 1.0 2,539.6 (86.3) (94.0) (1,961.0) 0.0
Ending balance (in shares) at Dec. 31, 2022 92,323,978     92,323,978          
Increase (Decrease) in Stockholders' Equity [Roll Forward]                  
Net loss $ (52.3) (43.1)           (43.1) (9.2)
Other comprehensive income (loss) (3.9) (3.9)         (3.9)    
Share-based compensation (in shares)       208,867          
Share-based compensation 3.3 3.3     3.3        
Conversion of preferred stock to common stock     (1,570) 157,019          
Noncontrolling interest of business acquired (2.3)               (2.3)
Ending balance (in shares) at Dec. 31, 2023     52,284            
Ending balance at Dec. 31, 2023 $ 344.1 $ 355.6 $ 0.0 $ 1.0 $ 2,542.9 $ (86.3) $ (97.9) $ (2,004.1) $ (11.5)
Ending balance (in shares) at Dec. 31, 2023 92,689,864     92,689,864          
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CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
CASH FLOWS FROM OPERATING ACTIVITIES:      
Net loss $ (52.3) $ (14.1) $ (167.1)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:      
Lower of cost or NRV inventory adjustment 0.0 39.6 0.0
Unrealized loss (gain) on derivative instruments 87.1 (201.5) 102.9
Depreciation, depletion and amortization 74.7 73.4 82.6
Change in deferred tax (benefit) provision (30.8) 44.2 (30.6)
Asset impairment 0.0 159.4 0.0
Other postretirement benefits gain, net 0.0 (8.9) 0.0
Loss on early extinguishment of debt 0.0 0.0 24.7
Other non-cash items - net 3.4 (22.8) (1.7)
Change in operating assets and liabilities, net of acquisition:      
Accounts receivable - net 36.9 13.7 (16.2)
Non-trade receivables 4.1 0.0 0.0
Manufacturing credit receivable (59.3) 0.0 0.0
Due from affiliates (15.5) 3.6 0.7
Inventories 25.8 (12.8) (134.5)
Prepaid and other current assets 2.9 5.6 (13.4)
Accounts payable, trade (19.4) (15.8) 44.8
Due to affiliates 51.7 (43.5) 38.3
Accrued and other current liabilities 0.0 8.8 5.0
Ravenswood retiree legal settlement (2.0) (2.0) (2.0)
PBGC settlement (4.5) (2.4) 0.0
Other - net 2.8 1.4 1.8
Net cash provided by (used in) operating activities 105.6 25.9 (64.7)
CASH FLOWS FROM INVESTING ACTIVITIES:      
Purchase of property, plant and equipment (95.0) (86.3) (83.0)
Proceeds from sale of property, plant and equipment 25.7 0.8 0.4
Acquisition of subsidiary net of cash acquired 11.5 0.0 0.0
Net cash used in investing activities (57.8) (85.5) (82.6)
CASH FLOWS FROM FINANCING ACTIVITIES:      
Repayment of Senior Notes due 2025 0.0 0.0 (250.0)
Early redemption and tender premiums paid 0.0 0.0 (18.1)
Proceeds from issuance of Senior Notes due 2028 0.0 0.0 250.0
Proceeds from issuance of Convertible Senior Notes 0.0 0.0 86.3
Repayments under Hawesville Term Loan 0.0 0.0 (20.0)
Debt issuance cost 0.0 (1.5) (7.4)
Purchase of capped calls related to Convertible Senior Notes 0.0 0.0 (5.7)
Carbon credit proceeds 36.8 0.0 0.0
Net cash (used in) provided by financing activities (13.0) 74.4 103.7
CHANGE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH 34.8 14.8 (43.6)
Cash, cash equivalents and restricted cash, beginning of year 55.5 40.7 84.3
Cash, cash equivalents and restricted cash, end of year 90.3 55.5 40.7
Cash paid for:      
Interest 35.2 27.0 36.8
Taxes, net of refunds 5.9 0.9 3.1
Non-cash investing activities:      
Capital expenditures 10.7 3.7 7.1
Capitalized interest 6.0 4.2 1.6
Vlissingen Facility Agreement      
CASH FLOWS FROM FINANCING ACTIVITIES:      
Borrowings under debt facilities 10.0 0.0 0.0
U.S. and Iceland Revolving Credit Facilities      
CASH FLOWS FROM FINANCING ACTIVITIES:      
Borrowings under revolving credit facilities 656.9 1,126.2 978.8
Repayments under revolving credit facilities (758.2) (1,114.8) (910.2)
Casthouse Facility      
CASH FLOWS FROM FINANCING ACTIVITIES:      
Borrowings under debt facilities 55.0 50.0 0.0
Iceland Term Facility      
CASH FLOWS FROM FINANCING ACTIVITIES:      
Borrowings under revolving credit facilities 0.0 14.5 0.0
Repayments under revolving credit facilities $ (13.5) $ 0.0 $ 0.0
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Summary of Significant Accounting Policies
 12 Months Ended
Dec. 31, 2023
Accounting Policies [Abstract]  
Summary of Significant Accounting Policies Summary of Significant Accounting Policies
Organization — Century Aluminum Company ("Century Aluminum," "Century," the "Company", "we", "us", "our" or "ours") is a holding company, whose principal subsidiaries are Century Kentucky, Inc. (together with its subsidiaries, "CAKY"), Nordural ehf ("Nordural"), Century Aluminum Sebree LLC ("Century Sebree") and Century Aluminum of South Carolina ("CASC"). CAKY operates a primary aluminum reduction facility in Hawesville, Kentucky ("Hawesville"). Nordural Grundartangi ehf, a subsidiary of Nordural, operates a primary aluminum reduction facility in Grundartangi, Iceland ("Grundartangi"). Century Sebree operates a primary aluminum reduction facility in Robards, Kentucky ("Sebree"). CASC operates a primary aluminum reduction facility in Goose Creek, South Carolina ("Mt. Holly").
In addition to our primary aluminum assets, we have a 55% joint venture interest in the Jamalco bauxite mining operation and alumina refinery in Jamaica ("Jamalco"). The Jamalco refinery supplies a substantial amount of the alumina used for production of primary aluminum at our aluminum smelter in Grundartangi, Iceland. Additionally, our subsidiary, Century Aluminum Vlissingen B.V., owns and operates a carbon anode production facility located in Vlissingen, the Netherlands ("Vlissingen"). Carbon anodes are used in the production of primary aluminum and Vlissingen currently supplies carbon anodes to Grundartangi.
As of December 31, 2023, Glencore owns 42.9% of Century’s outstanding common stock (46.0% on a fully-diluted basis assuming the conversion of all of the Series A Convertible Preferred Stock) and all of our outstanding Series A Convertible Preferred Stock. See Note 9. Shareholders' Equity for a full description of our outstanding Series A Convertible Preferred Stock. Century and Glencore enter into various transactions from time to time such as the purchase and sale of primary aluminum, purchase and sale of alumina and raw materials, tolling agreements as well as forward financial contracts and borrowing and other debt transactions. See Note 4. Related Party Transactions.
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.
The consolidated financial statements are prepared in accordance with generally accepted accounting principles in the United States ("U.S. GAAP"). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Certain prior period amounts included in current liabilities and non-current liabilities within the Consolidated Balance Sheets have been reclassified to conform with the current period presentation. These reclassifications had no impact on the Company's net loss, earnings per share, balance sheet or cash flows.
Our consolidated financial statements include the consolidated results of the Jamalco joint venture, an unincorporated joint venture between the Company and Clarendon Alumina Production Limited. Clarendon Alumina Production's interest in the joint venture is reflected as noncontrolling interest on the accompanying Consolidated Balance Sheet.
Revenue recognition — See Note 5. Revenue.
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 — These amounts are net of an allowance for expected losses of $0.5 million at both December 31, 2023 and 2022.
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 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 — 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 — 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 — 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 — 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 — 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.
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.
Unrealized gains on forward and derivative contracts are reported as part of cash flows from operations in the consolidated statements of cash flows.
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 — 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 — 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 — 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.
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 — 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.
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Acquisition of Jamalco
 12 Months Ended
Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]  
Acquisition of Jamalco Acquisition of Jamalco
On May 2, 2023, our wholly-owned subsidiary, Century Aluminum Jamaica Holdings, Inc., completed the acquisition of all the outstanding share capital of General Alumina Holdings Limited, the holder of a 55% interest in Jamalco JV ("Jamalco"), an unincorporated joint venture engaged in bauxite mining and alumina production in Jamaica. The remaining 45% interest in Jamalco is owned by Clarendon Alumina Production Limited ("CAP"), which in turn is owned by the Government of Jamaica. The purchase price for the acquisition was $1.00. The acquisition is expected to result in a bargain purchase gain in part due to the seller experiencing financial distress following curtailment of Jamalco's operations in the second half of 2021 due to a facility fire, with operations restarting in the second half of 2022.
The acquisition was accounted for as a business combination under the acquisition method of accounting. Determining the fair value of identified assets acquired, liabilities assumed and noncontrolling interest requires judgment and involves the use of significant estimates and assumptions. The Company based its fair value estimates on assumptions it believes to be reasonable but are inherently uncertain. These estimates and valuation of the property, plant and equipment, other assets, accrued and other current liabilities and long term liabilities as well as the related deferred bargain purchase gain and noncontrolling interest are preliminary as of December 31, 2023 and are subject to change as we finalize the valuation or if additional information about the facts and circumstances that existed at the acquisition date become available. We expect to finalize the purchase price allocation as soon as practicable within the measurement period, but not later than one year following the acquisition date.
The following table summarizes the estimated fair value of identified assets acquired, liabilities assumed and noncontrolling interest at the date of acquisition:
Preliminary purchase price allocationAmount
Identifiable assets acquired and liabilities assumed
Cash and cash equivalents$19.4 
Restricted cash8.3 
Accounts receivable - net7.7 
Non-trade receivables40.4 
Inventories103.9 
Prepaid and other current assets4.2 
Property, plant and equipment246.2 
Other assets26.0 
Accounts payable, trade(94.8)
Accrued and other current liabilities(30.8)
Other liabilities(35.5)
Asset retirement obligations(23.9)
Total identifiable assets acquired and liabilities assumed271.1 
Less: noncontrolling interest (2.3)
Deferred credit - preliminary bargain purchase gain273.4 
For the twelve months ended December 31, 2023, Jamalco contributed $150.3 million to our total revenues and a loss of $41.1 million to our total earnings. In connection with the acquisition, the Company incurred approximately $1.4 million of transaction costs for the twelve months ended December 31, 2023, which are included in selling, general and administrative expenses on the Consolidated Statements of Operations.
The following unaudited pro forma financial information reflects the results of operations of the Company for the twelve months ended December 31, 2023 and 2022, respectively, as if the acquisition of Jamalco had been completed on January 1, 2022. This unaudited pro forma financial information has been prepared for informational purposes and is not necessarily indicative of the actual consolidated results of operations had the acquisition been completed on January 1, 2022, nor is the information indicative of future results of operations of the combined companies.
Year ended December 31,
20232022
Revenue$2,235.1 $2,831.0 
Earnings$(45.9)$(33.1)
v3.24.0.1
Curtailment of Operations - Hawesville
 12 Months Ended
Dec. 31, 2023
Restructuring and Related Activities [Abstract]  
Curtailment of Operations - Hawesville Curtailment of Operations - Hawesville
In August 2022, we fully curtailed production at the Hawesville facility and expect to continue to maintain the plant with the intention of restarting operations when market conditions permit, including energy prices returning to more normalized levels and aluminum prices maintaining levels that can support the on-going costs and capital expenditures necessary to restart and operate the plant.
For the year ended December 31, 2023, we incurred curtailment charges of $16.6 million, including $9.0 million related to excess capacity charges. These charges were partially offset by income related to scrap and materials sales of $1.7 million. Comparatively, for the year ended December 31, 2022, we recognized an impairment charge of $159.4 million, approximately $18.1 million of expense during the year related to wages and severance triggered by our issuance of the WARN notice and excess capacity charges, partially offset by final plant idling activities. We also recognized a non-cash other postretirement
benefits ("OPEB") curtailment gain totaling $8.9 million. See Note 13. Pension and other postretirement benefits to the consolidated financial statements included herein for additional information.
v3.24.0.1
Related Party Transactions
 12 Months Ended
Dec. 31, 2023
Related Party Transactions [Abstract]  
Related Party Transactions Related Party Transactions
The significant related party transactions occurring during the years ended December 31, 2023, 2022 and 2021 are described below. We believe all of our transactions with related parties are at prices that approximate market.
Glencore ownership
As of December 31, 2023, Glencore plc and its affiliates (together "Glencore") beneficially owned 42.9% of Century’s outstanding common stock (46.0% on a fully-diluted basis assuming the conversion of all of the Series A Convertible Preferred Stock) and all of our outstanding Series A Convertible Preferred Stock. See Note 9. Shareholders' Equity for a full description of our outstanding Series A Convertible Preferred Stock. Century and Glencore enter into various transactions from time to time such as the purchase and sale of primary aluminum, purchase and sale of alumina and raw materials, tolling agreements as well as forward financial contracts and borrowing and other debt transactions.
Sales to Glencore
For the years ended December 31, 2023, 2022 and 2021 we derived approximately 73.8%, 60.2% and 60.0% of our consolidated sales from Glencore, respectively.
Glencore purchases aluminum produced at our U.S. smelters at prices based on the LME plus the Midwest regional delivery premium plus any additional market-based product premiums. Glencore purchases aluminum produced at our Grundartangi, Iceland smelter at prices based on the LME plus the European Duty Paid premium plus any additional market-based product premiums.
We have entered into agreements with Glencore pursuant to which we sell certain amounts of alumina at market-based prices. For the years ended December 31, 2023, 2022 and 2021 we recorded $191.7 million, $24.9 million, and $18.3 million of revenue related to alumina sales to Glencore, respectively.
Purchases from Glencore
We purchase a portion of our alumina and certain other raw material requirements from Glencore. Alumina purchases from Glencore during 2023 were priced based on published alumina and aluminum indices as well as fixed prices.
Financial contracts with Glencore
We have certain financial contracts with Glencore. See Note 20. Derivatives regarding these forward financial sales contracts.
Vlissingen Facility Agreement
On December 9, 2022, Vlissingen entered into a Facility Agreement with Glencore International AG pursuant to which Vlissingen may borrow from time to time up to $90 million (the “Vlissingen Facility Agreement”) in one or more loans at a fixed interest rate equal to 8.75% per annum and payable on December 2, 2024. See Note 8. Debt for additional information. Borrowings under the Facility Agreement are expected to be used for general corporate and working capital purposes of Century and its subsidiaries.
Carbon Credit Repurchase Agreement
On September 28, 2023, our wholly owned subsidiary Nordural Grundartangi ehf ("Grundartangi"), entered into a structured repurchase arrangement with an affiliate of Glencore pursuant to which it sold 390,000 European Union Allowances ("Carbon Credits") at a price of €82.18 per Carbon Credit, for an aggregate amount of €32.1 million. Pursuant to the terms of the transaction, Grundartangi will repurchase the same number of Carbon Credits at a price of €83.72 per Carbon Credit, for an aggregate amount of €32.7 million. On December 18, 2023, the repurchase arrangement was amended to extend the repurchase window and increase the repurchase price to €85.13 per Carbon Credit, for an aggregate amount of €33.2 million. Also on
December 18, 2023, Grundartangi sold an additional 40,000 Carbon Credits at a price of €69.30 per Carbon Credit and will repurchase the same number of Carbon Credits at a price of €70.71 per Carbon Credit for an aggregate amount of €2.8 million. Given the repurchase element of the agreement, the Company retains substantially all of the remaining benefits of the assets and has accounted for the transaction as a financing arrangement in accordance with Topic 606, Revenue from Contracts with Customers ("ASC 606").
Summary
A summary of the aforementioned significant related party sales and purchases for the years ended December 31, 2023, 2022 and 2021 is as follows:
 Year Ended December 31,
 202320222021
Net sales to Glencore$1,612.1 $1,671.1 $1,337.0 
Purchases from Glencore (1)
181.4 284.7 334.6 
(1)Includes settlements of financial contract positions.
v3.24.0.1
Revenue
 12 Months Ended
Dec. 31, 2023
Revenue from Contract with Customer [Abstract]  
Revenue Revenue
We disaggregate our revenue by geographical region as follows:
Year ended December 31,
Net Sales202320222021
United States$1,358.6 $1,737.2 $1,413.0 
Iceland826.8 1,040.1 799.5 
Total$2,185.4 $2,777.3 $2,212.5 
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.
Contract liabilities are recorded when cash payments are received or due in advance of performance. As of December 31, 2023, $30.6 million was recorded in Due to affiliates. There were no contract liabilities as of December 31, 2022.
v3.24.0.1
Leases
 12 Months Ended
Dec. 31, 2023
Leases [Abstract]  
Leases Leases
We are a lessee in various agreements for the lease of office space, land, automobiles, and mobile equipment. All of our leases are considered operating leases. The terms of our leases vary, including the lease term and the ability to renew or extend certain leases. As part of determining the lease term and potential extensions for purposes of calculating the ROUA and ROUL, we consider our historical practices related to renewal of certain leases. The weighted average remaining lease term for our operating leases was 11.8 years as of December 31, 2023 and 12.2 years as of December 31, 2022. Certain lease payment amounts are variable in nature and change periodically based on the local market consumer price index.
We use our incremental borrowing rate as the basis for the discount rate used to calculate the ROUA and ROUL, respectively, for our operating leases. The incremental borrowing rate is determined on a lease-by-lease basis and is based on the rate of interest that we would have to pay to borrow on a collateralized basis over a similar term for an amount equal to our lease payments. We consider the most likely financing options available for each lease based on the leased asset, legal entity party to the lease, economic environment in which the lease is denominated, the market conditions relative to the leased asset and our historical practices of obtaining financing for similar types of costs. The weighted average discount rate for our operating leases was 7.3% as of December 31, 2023 and 2022.
Our ROUA and ROUL balances for the years ended December 31, 2023 and December 31, 2022 were as follows (in millions):
December 31,
20232022
ROUA(1)
$24.7 $20.9 
ROUL - current(2)
$2.3 $1.8 
ROUL - non-current(3)
21.9 20.9 
Total ROUL$24.2 $22.7 
(1)ROUA was recorded as part of Other Assets within Non-current assets at December 31, 2023 and 2022.
(2)ROUL - current was recorded as part of Accrued and other current liabilities within Current liabilities at December 31, 2023 and 2022.
(3)ROUL - non-current was recorded as part of Other liabilities within Non-current liabilities at December 31, 2023 and 2022.
The undiscounted maturities of our operating lease liability balances as of December 31, 2023 are as follows (in millions):
YearDecember 31,
2024$3.9 
2025$3.7 
2026$3.3 
2027$2.9 
2028$2.9 
Thereafter$19.4 
Total $36.1 
Less: Interest$(11.9)
ROUL$24.2 
During 2023 and 2022, we entered into new lease obligations, which resulted in $3.2 million and $1.7 million of additional right of use assets. We acquired $2.4 million of right of use assets related to lease obligations assumed from the Jamalco acquisition.
Total operating expense includes the following (in millions):
December 31,
20232022
Operating leases expense$4.9 $4.5 
Short term lease expense0.6 0.4 
Total(1)
$5.5 $4.9 
(1)Total lease expense is included in cost of goods sold and selling, general, and administrative expenses on the Consolidated Statements of Operations.
We had cash outflows of $4.6 million and $4.1 million for amounts included in the ROUL balance at the beginning of the year related to our operating leases for the years ended December 31, 2023 and December 31, 2022, respectively.
v3.24.0.1
Fair Value Measurements
 12 Months Ended
Dec. 31, 2023
Fair Value Disclosures [Abstract]  
Fair Value Measurements Fair Value Measurements
We measure certain of our assets and liabilities at fair value. Fair value represents the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.
In general, reporting entities should apply valuation techniques to measure fair value that maximize the use of observable inputs and minimize the use of unobservable inputs. Observable inputs are developed using market data and reflect assumptions that market participants would use when pricing the asset or liability. Unobservable inputs are developed using the best information available about the assumptions and estimates that market participants would use when pricing the asset or liability.
The fair value hierarchy provides transparency regarding the inputs we use to measure fair value. We categorize each fair value measurement in its entirety into the following three levels, based on the lowest level input that is significant to the entire measurement:
•Level 1 Inputs – quoted prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity can access at the measurement date.
•Level 2 Inputs – inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.
•Level 3 Inputs – unobservable inputs for the asset or liability.
Recurring Fair Value Measurements
As of December 31, 2023
Level 1Level 2Level 3Total
ASSETS:    
Cash equivalents$16.8 $— $— $16.8 
Trust assets (1)
0.2— — 0.2
Derivative instruments— 2.9 — 2.9 
TOTAL$17.0 $2.9 $— $19.9 
LIABILITIES:    
Derivative instruments— 7.9 — 7.9 
TOTAL$— $7.9 $— $7.9 
Recurring Fair Value Measurements
As of December 31, 2022
Level 1Level 2Level 3Total
ASSETS:    
Cash equivalents$5.6 $— $— $5.6 
Trust assets (1)
0.1— — 0.1 
Derivative instruments— 127.3 1.8 129.1 
TOTAL$5.7 $127.3 $1.8 $134.8 
LIABILITIES:    
Contingent obligation – net (2)
$— $— $— $— 
Derivative instruments— 26.4 4.6 31.0 
TOTAL$— $26.4 $4.6 $31.0 
(1)Trust assets are currently invested in money market funds. These trust assets are held to fund the non-qualified supplemental executive pension benefit obligations for certain of our officers.
(2)See Note 17. Commitments and Contingencies for additional information about the contingent obligation.
The following section describes the valuation techniques and inputs used for fair value measurements categorized within Level 2 or Level 3 of the fair value hierarchy:
Level 2 Fair Value Measurements:
Asset / LiabilityValuation TechniquesInputs
LME forward financial sales contractsDiscounted cash flowsQuoted LME forward market
Fixed for floating swapsDiscounted cash flowsQuoted LME forward market, quoted MWP forward market
Nord Pool power price swaps Discounted cash flowsQuoted Nord Pool forward market
Indiana Hub power price swapsDiscounted cash flowsQuoted Indiana Hub forward market
FX swaps Discounted cash flowsEuro/USD forward exchange rate
Casthouse currency hedgesDiscounted cash flowsEuro/USD forward exchange rate; ISK/USD forward exchange rate
Heavy Fuel Oil ("HFO") price swapsDiscounted cash flowsQuoted HFO forward market
When valuing Level 3 assets and liabilities, we use certain significant unobservable inputs. Management incorporates various inputs and assumptions including forward commodity prices, commodity price volatility, and macroeconomic conditions, including interest rates and discount rates. Our estimates of significant unobservable inputs are ultimately based on our estimates of risks that market participants would consider when valuing our assets and liabilities. As of the year ending, December 31, 2023, there were no level three assets and liabilities.
The following table presents the inputs for fair value measurements categorized within Level 3 of the fair value hierarchy, along with information regarding significant unobservable inputs used to value Level 3 assets and liabilities:






Level 3 Fair Value Measurements:
As of December 31, 2023
As of December 31, 2022
Asset / LiabilityValuation TechniqueObservable InputsSignificant Unobservable InputFair Value Value/Range of Unobservable InputFair ValueValue/Range of Unobservable Input
LME forward financial sales contractsDiscounted cash flowsQuoted LME forward market
Discount rate net (1)
$— 8.58%$(2.8)8.58%
(1) Represents risk adjusted discount rate
The following table presents the fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis.
For the twelve months ended December 31, 2023
Level 3 AssetsLevel 3 Liabilities
LME forward financial sales contractsLME forward financial sales contracts
Balance as of January 1, 2023
$1.8 $(4.6)
Transfers out of Level 3 (1)
(1.8)4.6 
Balance as of December 31, 2023
$— $— 
Change in unrealized gains (losses) (2)
$— $— 
(1)Transfer out of Level 3 due to period of time remaining in derivative contract.
(2)Gains and losses are presented in the Consolidated Statement of Operations within the line item "Net gain (loss) on forward and derivative contracts."
v3.24.0.1
Debt
 12 Months Ended
Dec. 31, 2023
Debt Disclosure [Abstract]  
Debt Debt
 December 31,
 20232022
Debt classified as current liabilities:  
Hancock County industrial revenue bonds ("IRBs") due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) (1)
$7.8 $7.8 
U.S. Revolving Credit Facility (2)
23.7 90.0 
Iceland Revolving Credit Facility (3)
— 35.0 
Grundartangi Casthouse Facility (4)
5.5 — 
Iceland Term Facility (5)
1.3 13.3 
Vlissingen Facility Agreement (6)
10.0 — 
Debt classified as non-current liabilities:  
Grundartangi casthouse facility, net of financing fees of $0.7 million at December 31, 2023 (4)
98.8 49.4 
Iceland Term Facility, net of financing fees of $0.0 million and current portion at December 31, 2023 (5)
— 1.2 
7.5% senior secured notes due April 1, 2028, net of financing fees of $2.6 million at December 31, 2023, interest payable semiannually
247.4 246.6 
2.75% convertible senior notes due May 1, 2028, net of financing fees of $1.5 million at December 31, 2023, interest payable semiannually
84.7 84.4 
Total$479.2 $527.7 
(1)The IRBs are classified as current liabilities because they are remarketed weekly and could be required to be repaid upon demand if there is a failed remarketing. The interest rate at December 31, 2023 was 4.00%.
(2)We incur interest at a base rate plus applicable margin as defined within the agreement. The interest rate at December 31, 2023 was 9.25%.
(3)We incur interest at a base rate plus applicable margin as defined within the agreement. The were no outstanding borrowings at December 31, 2023.
(4)We incur interest at a base rate plus applicable margin as defined within the agreement. The interest rate at December 31, 2023 was 8.86%.
(5)We incur interest at a rate equal to 3.2% plus EUR EURIBOR 1 month as published by the European Money Markets Institute as defined within the agreement. The interest rate at December 31, 2023 was 7.05%.
(6)We incur interest at a fixed rate of 8.75%.
7.5% Senior Secured Notes due 2028
General. On April 14, 2021, we issued $250.0 million in aggregate principal amount of 7.5% senior secured notes due 2028 (the "2028 Notes"). We received proceeds of $245.2 million, after payment of certain financing fees and related expenses.
Interest Rate. The 2028 Notes bear interest semi-annually in arrears on April 1 and October 1 of each year, beginning on October 1, 2021, at a rate of 7.5% per annum in cash.
Maturity. The 2028 Notes mature on April 1, 2028.
Seniority. The 2028 Notes are senior secured obligations of Century, ranking equally in right of payment with all existing and future senior indebtedness of Century, but effectively senior to unsecured debt to the extent of the value of collateral.
Guaranty. Our obligations under the 2028 Notes are guaranteed by all of our existing and future domestic restricted subsidiaries (the “Guarantor Subsidiaries”), except for foreign owned holding companies, any domestic restricted subsidiary that owns no assets other than equity interests or other investments in foreign subsidiaries and certain immaterial subsidiaries, which guaranty shall in each case be a senior secured obligation of such Guarantor Subsidiaries, ranking equally in right of payment with all existing and future senior indebtedness of such Guarantor Subsidiaries but effectively senior to unsecured debt to the extent of the value of collateral.
Collateral. Our obligations under the 2028 Notes and the Guarantor Subsidiaries' obligations under the guarantees are secured by a pledge of and lien on (subject to certain exceptions):
(i) all of our and the Guarantor Subsidiaries' property, plant and equipment (other than certain excluded property);
(ii) all equity interests in subsidiaries directly owned by Century or any Guarantor Subsidiaries; and
(iii) proceeds of the foregoing.
Under certain circumstances, the indenture and the security documents governing the 2028 Notes will permit us and the Guarantors to incur additional debt that also may be secured by liens on the collateral that are equal to or have priority over the liens securing the 2028 Notes. The collateral agent for the 2028 Notes will agree with the collateral agent for the other debt holders and us under such circumstances to enter into an intercreditor agreement that will cause the liens securing the 2028 Notes to be contractually subordinated to the liens securing such additional debt.
Redemption Rights. Prior to April 1, 2024, we may redeem the 2028 Notes, in whole or in part, at a redemption price equal to 100.00% of the principal amount plus a make-whole premium and accrued and unpaid interest, and if redeemed during the twelve-month period beginning on April 1 of the years indicated below, at the following redemption prices plus accrued and unpaid interest:
YearPercentage
2024103.750%
2025101.875%
2026 and Thereafter100.000%
Upon a change of control (as defined in the indenture governing the 2028 Notes), we will be required to make an offer to purchase the 2028 Notes at a purchase price equal to 101% of the outstanding principal amount of the 2028 Notes on the date of the purchase, plus accrued and unpaid interest to, but not including, the date of purchase.
Covenants. The indenture governing the 2028 Notes contains customary covenants which may limit our ability, and the ability of certain of our subsidiaries, to: (i) incur additional debt; (ii) incur additional liens; (iii) pay dividends or make distributions in respect of capital stock; (iv) purchase or redeem capital stock; (v) make investments or certain other restricted payments; (vi) sell assets; (vii) issue or sell stock of certain subsidiaries; (viii) enter into transactions with shareholders or affiliates; and (ix) effect a consolidation or merger.
Fair Value. As of December 31, 2023, the total estimated fair value of the 2028 Notes was $243.5 million. Although we use quoted market prices for identical debt instruments, the markets on which they trade are not considered to be active and are therefore considered Level 2 fair value measurements.
2.75% Convertible Notes due 2028
General. On April 9, 2021, we completed a private offering of $86.3 million aggregate principal amount of convertible senior notes due 2028 (the "Convertible Notes"). The Convertible Notes were issued at a price of 100% of their aggregate principal amount. We received proceeds of $83.7 million, after payment of certain financing fees and related expenses.
The initial conversion rate for the Convertible Notes is 53.3547 shares of the Company's common stock per $1,000 principal amount of Convertible Notes, which is equivalent to an initial conversion price of approximately $18.74 per share of the Company's common stock. The conversion rate and conversion price are subject to customary adjustments under certain circumstances in accordance with the terms of the indenture.
Interest Rate. The Convertible Notes will bear interest semi-annually in arrears on May 1 and November 1 of each year, beginning on November 1, 2021, at a rate of 2.75% per annum in cash.
Maturity. The Convertible Notes will mature on May 1, 2028, unless earlier converted, repurchased, or redeemed.
Seniority. The Convertible Notes are the Company’s senior unsecured obligations and rank senior in right of payment to any of the Company’s indebtedness that is expressly subordinated in right of payment to the Convertible Notes; equal in right of payment to any of the Company’s unsecured indebtedness that is not so subordinated; effectively junior in right of payment to any of the Company’s senior secured indebtedness to the extent of the value of the assets securing such indebtedness; and structurally junior to all indebtedness and other liabilities (including trade payables) of the Company’s subsidiaries.
Redemption rights. We may not redeem the Convertible Notes prior to May 6, 2025. On or after May 6, 2025, we may redeem for cash all or part of the Convertible Notes at our option if the last reported sale price of our common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading-day period (including the last trading day of such period) ending on and including the trading day immediately preceding the date on which we provide notice of redemption, at a redemption price equal to 100% of the principal amount of the Convertible Notes to be redeemed, plus accrued and unpaid interest.
Upon conversion, we may satisfy our conversion obligation by paying or delivering, as applicable, cash, shares of our common stock or a combination of cash and shares of our common stock, at our election, based on the applicable conversion rate. In addition, if certain corporate events that constitute a make-whole fundamental change (as defined in the indenture) occur, then the conversion rate will, in certain circumstances, be increased for a specified period of time. Additionally, in the event of a corporate event constituting a fundamental change (as defined in the indenture), holders of the Convertible Notes may require us to repurchase all or a portion of their Convertible Notes at a repurchase price equal to 100% of the principal amount of the Convertible Notes being repurchased, plus accrued and unpaid interest to, but excluding, the date of the fundamental change repurchase.
As of December 31, 2023, the if-converted value of the Convertible Notes did not exceed the outstanding principal amount.
Fair Value. As of December 31, 2023, the total estimated fair value of the Convertible Notes was $75.3 million. Although we use quoted market prices for identical debt instruments, the markets on which they trade are not considered to be active and are therefore considered Level 2 fair value measurements.
U.S. Revolving Credit Facility
General. We and certain of our direct and indirect domestic subsidiaries (the "Borrowers") have a senior secured revolving credit facility with a syndicate of lenders (the "U.S. revolving credit facility"). On June 14, 2022 we amended our U.S. revolving credit facility, increasing our borrowing capacity to $250.0 million, including up to $150.0 million under a letter of credit sub-facility. The U.S. revolving credit facility matures on June 14, 2027.
Any letters of credit issued and outstanding under the U.S. revolving credit facility reduce our borrowing availability on a dollar-for-dollar basis. At December 31, 2023, there were $23.7 million outstanding borrowings and $61.4 million of outstanding letters of credit issued under our U.S. revolving credit facility. Principal payments, if any, are due upon maturity of the U.S. revolving credit facility and may be prepaid without penalty.
Status of our U.S. revolving credit facility:
December 31, 2023
Credit facility maximum amount$250.0 
Borrowing availability128.8 
Outstanding letters of credit issued61.4 
Outstanding borrowings23.7 
Borrowing availability, net of outstanding letters of credit and borrowings43.7 
Borrowing Base. The availability of funds under the U.S. revolving credit facility is limited by a specified borrowing base consisting of the Borrower's accounts receivable and inventory which meet the eligibility criteria.
Guaranty. The Borrowers' obligations under the U.S. revolving credit facility are guaranteed by certain of our domestic subsidiaries and secured by a continuing lien upon and a security interest in all of the Borrowers' accounts receivable, inventory and certain bank accounts. Each Borrower is liable for any and all obligations under the U.S. revolving credit facility on a joint and several basis.
Interest Rates and Fees. Any amounts outstanding under the U.S. revolving credit facility will bear interest at our option of either the secured overnight financing rate ("SOFR") or a base rate, plus, in each case, an applicable interest margin. The applicable interest margin is determined based on the average daily availability for the immediately preceding quarter. In addition, we pay an unused line fee on undrawn amounts, less the amount of our letters of credit exposure. For standby letters of credit, we are required to pay a fee on the face amount of such letters of credit that varies depending on whether the letter of credit exposure is cash collateralized.
Prepayments. We can make prepayments of amounts outstanding under the U.S. revolving credit facility, in whole or in part, without premium or penalty, subject to standard breakage costs, if applicable. We may be required to apply the proceeds from sales of collateral accounts, other than sales of inventory in the ordinary course of business, to repay amounts outstanding under the revolving credit facility and correspondingly reduce the commitments there under.
Covenants. The U.S. revolving credit facility contains customary covenants, including restrictions on mergers and acquisitions, indebtedness, affiliate transactions, liens, dividends and distributions, dispositions of collateral, investments, and prepayments of indebtedness, as well as a covenant that requires the Borrowers to maintain certain minimum liquidity or availability requirements.
Events of Default. The U.S. revolving credit facility also includes customary events of default, including nonpayment, misrepresentation, breach of covenant, bankruptcy, change of ownership, certain judgments and certain cross defaults. Upon the occurrence of an event of default, commitments under the U.S. revolving credit facility may be terminated and amounts outstanding may be accelerated and declared immediately due and payable.
Iceland Revolving Credit Facility
General. Our wholly-owned subsidiary, Nordural Grundartangi ehf ("Grundartangi"), entered into a revolving credit facility agreement with Landsbankinn hf., dated November 2013, as amended (the "Iceland revolving credit facility") which originally provided for borrowings of up to $50.0 million in the aggregate. On February 4, 2022, we amended the Iceland revolving credit facility and increased the facility amount to $80.0 million in the aggregate. On September 28, 2022, we further amended the Iceland revolving credit facility and increased the facility amount to $100.0 million in aggregate. Under the terms of the Iceland revolving credit facility, when Grundartangi borrows funds it will designate a repayment date, which may be any date prior to the maturity of the Iceland revolving credit facility. At December 31, 2023, there were no outstanding borrowings under our Iceland revolving credit facility. The Iceland revolving credit facility has a term through December 2026.
Status of our Iceland revolving credit facility:
December 31, 2023
Credit facility maximum amount$100.0 
Borrowing availability100.0 
Outstanding letters of credit issued— 
Outstanding borrowings— 
Borrowing availability, net of outstanding letters of credit and borrowings100.0 
Borrowing Base. The availability of funds under the Iceland revolving credit facility is limited by a specified borrowing base consisting of inventory and accounts receivable of Grundartangi.
Security. Grundartangi's obligations under the Iceland revolving credit facility are secured by a general bond under which Grundartangi's inventory and accounts receivable are pledged to secure full payment of the loan.
Interest Rates and Fees. Any amounts outstanding under the Iceland revolving credit facility will bear interest at SOFR plus a margin per annum.
Prepayments. Any outstanding borrowings may be prepaid without penalty or premium in whole or in part.
Covenants. The Iceland revolving credit facility contains customary covenants, including restrictions on mergers and acquisitions, dispositions of assets, compliance with permits, laws and payment of taxes, as well as a covenant that requires Grundartangi to maintain a certain minimum equity ratio.
Events of Default. The Iceland revolving credit facility also includes customary events of default, including nonpayment, loss of license, cessation of operations, unlawfulness, breach of covenant, bankruptcy, change of ownership, certain judgments and certain cross defaults. Upon the occurrence of an event of default, commitments under the Iceland revolving credit facility may be terminated and amounts outstanding may be accelerated and declared immediately due and payable.
Grundartangi Casthouse Facility
On November 2, 2021, in connection with the casthouse project at Grundartangi, we entered into an eight-year Term Facility Agreement with Arion Bank hf, to provide for borrowings up to $130.0 million (the "Casthouse Facility"). Under the Casthouse Facility, repayments of principal amounts will be made in equal quarterly installments equal to 1.739% of the principal amount, the first payment occurring in July 2024, with the remaining 60% of the principal amount to be paid no later than the termination date in December 2029. As of December 31, 2023, there were $104.3 million in outstanding borrowings under the Casthouse Facility.
Security. Grundartangi's obligations under the Casthouse Facility are secured by a general bond on an aggregate of $430.0 million in assets and rights related to Grundartangi.
Interest Rates and Fees. The interest rate shall be at a base rate plus the applicable margin as set forth in the agreement. Grundartangi shall pay an arrangement fee equal to 0.78% of the total facility amount, 50% of which was paid upfront and 50% to be paid at the end of the availability period, and shall pay a commitment fee of 0.38% per annum on undrawn commitments, payable quarterly at the same time as interest payments are due and payable.
Prepayments. We can make prepayments of amounts outstanding under the Casthouse Facility, in whole or in part, without premium or penalty, together with accrued interest on the amount prepaid and subject to standard breakage costs, if applicable.
Covenants. The Casthouse Facility contains customary covenants, including restrictions on mergers and acquisitions, indebtedness, preservation of assets, and dispositions of assets, as well as a covenant that requires Grundartangi to maintain a certain minimum equity ratio.
Events of Default. The Casthouse Facility also includes customary events of default, including nonpayment, loss of license, cessation of operations, unlawfulness, breach of covenant, bankruptcy, change of ownership, certain judgments and certain cross defaults. Upon the occurrence of an event of default, commitments under the Casthouse facility may be terminated and amounts outstanding may be accelerated and declared immediately due and payable.
Iceland Term Facility
Our wholly-owned subsidiary, Grundartangi, has entered into a Term Facility Agreement with Arion Bank hf, dated September 2022, (the "Iceland Term Facility") to provide for borrowings up to €13.6 million. Repayments of principal amounts will be made in equal monthly installments, the first payment occurring in February 2023, with the remainder of the principal amount to be paid no later than the termination date in January 2024. Borrowings under the Iceland Term Facility will bear
interest at a rate equal to 3.2% plus EUR EURIBOR 1 month as published by the European Money Markets Institute. As of December 31, 2023, there were $1.3 million (€1.1 million) in outstanding borrowings under the Iceland Term Facility.
Vlissingen Facility Agreement
On December 9, 2022, Vlissingen entered into a Facility Agreement with Glencore International AG pursuant to which Vlissingen may borrow from time to time up to $90 million (the “Vlissingen Facility Agreement”) in one or more loans payable on December 2, 2024, the maturity date of the Facility Agreement. As of December 31, 2023, there was $10.0 million in outstanding borrowings under the Vlissingen Facility Agreement.
Security. Vlissingen’s obligations under the Vlissingen Facility Agreement are secured by liens on the ground lease on which Vlissingen’s facilities are located, Vlissingen’s moveable assets, financial assets, receivables and other assets, and Vlissingen’s shares.
Interest Rates and Fees. Any amounts outstanding under the Vlissingen Facility Agreement will bear interest at a fixed interest rate equal to 8.75% per annum and payable on December 2, 2024
Prepayments. Any outstanding borrowings may be prepaid without penalty or premium in whole or in part without any charge, fee premium or penalty.
Covenants. The Vlissingen Facility Agreement contains customary covenants including with respect to mergers, guarantees and preservation and dispositions of assets.
Events of Default. The Vlissingen Facility Agreement also includes customary events of default, including nonpayment, breach of any provision or representation under the agreement, and certain cross-default and insolvency events. Upon the occurrence of an event of default, commitments under the Vlissingen Facility Agreement may be terminated and amounts outstanding may be accelerated and declared immediately due and payable.
Hancock County Industrial Revenue Bonds
As part of the purchase price for our acquisition of the Hawesville facility, we assumed IRBs which were issued in connection with the financing of certain solid waste disposal facilities constructed at the Hawesville facility. The IRBs bear interest at a variable rate not to exceed 12% per annum determined weekly based upon prevailing rates for similar bonds in the industrial revenue bond market and interest on the IRBs is paid quarterly. The IRBs are secured by a letter of credit issued under our U.S revolving credit facility and mature in April 2028.
Surety Bond Facility
As part of our normal business operations, we are required to provide surety bonds or issue letters of credit in certain states in which we do business as collateral for certain workers' compensation obligations. In June 2022, we entered into a surety bond facility with an insurance company to provide such bonds when applicable. As of December 31, 2023, we had issued surety bonds totaling $6.6 million. As we had previously guaranteed our workers' compensation obligations through issuance of letters of credit against our revolving credit facility, the surety bond issuance increases credit facility availability.
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Shareholders' Equity
 12 Months Ended
Dec. 31, 2023
Stockholders' Equity Note [Abstract]  
Shareholders' Equity Shareholders' Equity
Common Stock
As of December 31, 2023 and 2022, we had 195,000,000 shares of common stock, $0.01 cent par value, authorized under our Restated Certificate of Incorporation, of which 99,876,385 shares were issued and 92,689,864 shares were outstanding at December 31, 2023; 99,510,499 shares were issued and 92,323,978 shares were outstanding at December 31, 2022.
The rights, preferences and privileges of holders of our common stock are subject to, and may be adversely affected by, the rights of the holders of shares of any series of our preferred stock which are currently outstanding, including our Series A Convertible Preferred Stock, or which we may designate and issue in the future.
Preferred Stock
As of December 31, 2023 and 2022, we had 5,000,000 shares of preferred stock, $0.01 cent par value per share, authorized under our Restated Certificate of Incorporation. Our Board of Directors may issue preferred stock in one or more series and determine for each series the dividend rights, conversion rights, voting rights, redemption rights, liquidation preferences, sinking fund terms and the number of shares constituting that series, as well as the designation thereof. Depending upon the terms of preferred stock established by our Board of Directors, any or all of the preferred stock could have preference over the common stock with respect to dividends and other distributions and upon the liquidation of Century. In addition, issuance of any shares of preferred stock with voting powers may dilute the voting power of the outstanding common stock.
Series A Convertible Preferred Stock
Shares Authorized and Outstanding. In 2008, we issued 160,000 shares of our Series A Convertible Preferred Stock. Glencore holds all of the issued and outstanding Series A Convertible Preferred Stock. At December 31, 2023 and December 31, 2022, 52,284 shares and 53,854 shares were outstanding, respectively.
The issuance of common stock under our stock incentive programs, debt exchange transactions and any stock offering that excludes Glencore participation triggers anti-dilution provisions of the preferred stock agreement and results in the automatic conversion of Series A Convertible Preferred Stock shares into shares of common stock. The conversion of preferred to common shares is 100 shares of common for each share of preferred stock. Our Series A Convertible Preferred Stock has a par value of $0.01 per share.
Dividend Rights. So long as any shares of our Series A Convertible Preferred Stock are outstanding, we may not pay or declare any dividend or make any distribution upon or in respect of our common stock or any other capital stock ranking, on a parity with or junior to, the Series A Convertible Preferred Stock in respect of dividends or liquidation preference, unless we, at the same time, declare and pay a dividend or distribution on the shares of Series A Convertible Preferred Stock (a) in an amount equal to the amount such holders would receive if they were the holders of the number of shares of our common stock into which their shares of Series A Convertible Preferred Stock are convertible as of the record date fixed for such dividend or distribution, or (b) in the case of a dividend or distribution on other capital stock ranking on a parity with or junior to the Series A Convertible Preferred Stock in such amount and in such form as (based on the determination of holders of a majority of the Series A Convertible Preferred Stock) will preserve, without dilution, the economic position of the Series A Convertible Preferred Stock relative to such other capital stock.
Voting Rights. The Series A Convertible Preferred Stock has no voting rights for the election of directors or on other matters where the shares of common stock have voting rights. However, we may not change the powers, preferences, or rights given to the Series A Convertible Preferred Stock, or authorize, create or issue any additional shares of Series A Convertible Preferred Stock without the affirmative vote of the holders of a majority of the shares of Series A Convertible Preferred Stock then outstanding (voting separately as a class).
Liquidation Rights. Upon any liquidation, dissolution, or winding-up of Century, the holders of shares of Series A Convertible Preferred Stock are entitled to receive a preferential distribution of $0.01 per share out of the assets available for distribution. In addition, upon any liquidation, dissolution or winding-up of Century, if our assets are sufficient to make any distribution to the holders of the common stock, then the holders of shares of Series A Convertible Preferred Stock are also entitled to share ratably with the holders of common stock in the distribution of Century’s assets (as though the holders of Series A Convertible Preferred Stock were holders of that number of shares of common stock into which their shares of Series A Convertible Preferred Stock are convertible). However, the amount of any such distribution will be reduced by the amount of the preferential distribution received by the holders of the Series A Convertible Preferred Stock.
Transfer Restrictions. Glencore is prohibited from transferring shares of Series A Convertible Preferred Stock to any party other than an affiliate who agrees to become bound by certain agreements associated with these shares.
Automatic Conversion. The Series A Convertible Preferred Stock automatically converts, without any further act of Century or any holders of Series A Convertible Preferred Stock, into shares of common stock, at a conversion ratio of 100 shares of common stock for each share of Series A Convertible Preferred Stock, upon the occurrence of any of the following automatic conversion events:
•If we sell or issue shares of common stock or any other stock that votes generally with our common stock, or the occurrence of any other event, including a sale, transfer or other disposition of common stock by Glencore, as a result of which the percentage of voting stock held by Glencore decreases, an amount of Series A Convertible Preferred Stock will convert to common stock to restore Glencore to its previous ownership percentage;
•If shares of Series A Convertible Preferred Stock are transferred to an entity that is not an affiliate of Glencore, such shares of Series A Convertible Preferred Stock will convert to shares of our common stock, provided that such transfers may only be made pursuant to an effective registration statement;
•Upon a sale of Series A Convertible Preferred Stock by Glencore in a Rule 144 transaction in which the shares of Series A Convertible Preferred Stock and our common stock issuable upon the conversion thereof are not directed to any purchaser, such shares of Series A Convertible Preferred Stock sold will convert to shares of our common stock; and
•Immediately prior to and conditioned upon the consummation of a merger, reorganization or consolidation to which we are a party or a sale, abandonment, transfer, lease, license, mortgage, exchange or other disposition of all or substantially all of our property or assets, in one or a series of transactions where, in any such case, all of our common stock would be converted into the right to receive, or exchanged for, cash and/or securities, other than any transaction in which the Series A Convertible Preferred Stock will be redeemed.
Optional Conversion. Glencore has the option to convert the Series A Convertible Preferred Stock in a tender offer or exchange offer, at the same conversion ratio as above, in which a majority of the outstanding shares of our common stock have been tendered by the holders thereof and not duly withdrawn at the expiration time of such tender or exchange offer, so long as the Series A Convertible Preferred Stock is tendered or exchanged in such offer.
Stock Combinations – Adjustments. If, at any time while the Series A Convertible Preferred Stock is outstanding, Century combines outstanding common stock into a smaller number of shares, then the number of shares of common stock issuable on conversion of each share of Series A Convertible Preferred Stock will be decreased in proportion to such decrease in the aggregate number of shares of common stock outstanding.
Redemptions or Repurchases of Common Stock. We may not redeem or repurchase our common stock unless we redeem or repurchase, or otherwise make a payment on, a pro-rata number of shares of the Series A Convertible Preferred Stock. These restrictions do not apply to our open market repurchases or our repurchases pursuant to our employee benefit plans.
Right of Redemption. The Series A Convertible Preferred Stock will be redeemed by Century if any of the following events occur (at a redemption price based on the trading price of our common stock prior to the announcement of such event) and Glencore votes its shares of our common stock in opposition to such events:
•We propose a merger, reorganization or consolidation, sale, abandonment, transfer, lease, license, mortgage, exchange or other disposition of all or substantially all of our property or assets where any of our common stock would be converted into the right to receive, or exchanged for, assets other than cash and/or securities traded on a national stock exchange or that are otherwise readily marketable, or
•We propose to dissolve and wind up operations and any assets, other than cash and/or securities traded on a national stock exchange or that are otherwise readily marketable, are to be distributed to the holders of our common stock.
Stock Repurchase Program
In 2011, our Board of Directors authorized a $60.0 million stock repurchase program and during the first quarter of 2015, our Board of Directors increased the size of the program by $70.0 million. Under the program, Century is authorized to repurchase up to $130.0 million of our outstanding shares of common stock, from time to time, on the open market at prevailing market prices, in block trades or otherwise. The timing and amount of any shares repurchased will be determined by our management based on its evaluation of market conditions, the trading price of our common stock and other factors. The stock repurchase program may be suspended or discontinued at any time.
Shares of common stock repurchased are recorded at cost as treasury stock and result in a reduction of shareholders’ equity in the consolidated balance sheets. From time to time, treasury shares may be reissued as contributions to our employee
benefit plans and for the conversion of preferred stock. When shares are reissued, we use an average cost method for determining cost. The difference between the cost of the shares and the reissuance price is added to or deducted from additional paid-in capital.
Through December 31, 2023, we repurchased 7,186,521 shares of common stock for an aggregate purchase price of $86.3 million. We have made no repurchases since April 2015 and have approximately $43.7 million remaining under the repurchase program authorization as of December 31, 2023.
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Inventories
 12 Months Ended
Dec. 31, 2023
Inventory, Net [Abstract]  
Inventories Inventories
Inventories, at December 31, consist of the following:
 20232022
Raw materials$162.5 $64.9 
Work-in-process42.9 46.0 
Finished goods46.3 58.0 
Operating and other supplies225.3 229.9 
Inventories$477.0 $398.8 
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Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2023
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment Property, Plant and Equipment
Property, plant and equipment, at December 31, consist of the following:
 20232022
Land and improvements$105.1 $39.8 
Mineral Reserves57.6 — 
Buildings and improvements328.8 305.7 
Machinery and equipment1,580.7 1,495.3 
Construction in progress160.3 59.0 
 2,232.5 1,899.8 
Less accumulated depreciation, amortization and depletion(1,228.3)(1,155.4)
Property, plant and equipment - net$1,004.2 $744.4 
For the years ended December 31, 2023, 2022 and 2021, we recorded depreciation, amortization and depletion expense of $74.7 million, $73.4 million, and $82.6 million, respectively.
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Accumulated Other Comprehensive Loss (AOCL)
 12 Months Ended
Dec. 31, 2023
Additional financial information disclosures [Abstract]  
Accumulated Other Comprehensive Loss (AOCL) Accumulated Other Comprehensive Loss ("AOCL")
Components of AOCL20232022
Defined benefit plan liabilities$(101.8)$(98.0)
Unrealized gain on financial instruments1.6 1.7 
Other comprehensive loss before income tax effect(100.2)(96.3)
Income tax effect(1)
2.3 2.3 
Accumulated other comprehensive loss$(97.9)$(94.0)
(1)The allocation of the income tax effect to the components of other comprehensive loss is as follows:
20232022
Defined benefit plan liabilities$2.6 $2.6 
Unrealized loss on financial instruments(0.3)(0.3)
The following table summarizes the changes in the accumulated balances for each component of AOCL:
Defined benefit plan and other postretirement liabilitiesUnrealized gain (loss) on financial instrumentsTotal, net of tax
Balance, December 31, 2020
$(120.6)$1.8 $(118.8)
Other comprehensive income (loss) before reclassifications31.6 — 31.6 
Net amount reclassified to net income (loss)5.0 (0.1)4.9 
Balance, December 31, 2021
(84.0)1.7 (82.3)
Other comprehensive income (loss) before reclassifications(5.9)— (5.9)
Net amount reclassified to net income (loss)(5.7)(0.1)(5.8)
Balance, December 31, 2022
(95.6)1.6 (94.0)
Other comprehensive income (loss) before reclassifications(10.1)— (10.1)
Net amount reclassified to net income (loss)6.3 (0.1)6.2 
Balance, December 31, 2023
$(99.4)$1.5 $(97.9)
Reclassifications out of AOCL were included in the consolidated statements of operations as follows:
Year Ended December 31,
AOCL ComponentsLocation202320222021
Defined benefit plan and other postretirement liabilitiesCost of goods sold$(1.0)$3.5 $26.8 
Other income, net— (8.9)— 
Selling, general and administrative expenses(0.2)3.3 2.9 
Other operating expense (income), net(2.6)(9.0)7.3 
Income tax expense— (0.3)(0.4)
Net of tax$(3.8)$(11.4)$36.6 
Gain (loss) on financial instrumentsCost of goods sold$(0.1)$(0.2)$(0.1)
Income tax effect $— (0.1)0.0 
Net of tax$(0.1)$(0.3)$(0.1)
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Pension and Other Postretirement Benefits
 12 Months Ended
Dec. 31, 2023
Retirement Benefits [Abstract]  
Pension and Other Postretirement Benefits Pension and Other Postretirement Benefits 
Pension Benefits
We maintain noncontributory defined benefit pension plans for certain domestic hourly and salaried employees. For the eligible domestic salaried employees, plan benefits are based primarily on years of service and average compensation during the later years of employment. For hourly employees, plan benefits are based primarily on a formula that provides a specific benefit for each year of service. Our funding policy is to contribute amounts based upon actuarial and economic assumptions designed to achieve adequate funding of the projected benefit obligations and to meet the minimum funding requirements of the Employee Retirement Income Security Act of 1974 ("ERISA"). In addition, we maintain the supplemental executive retirement benefit ("SERB") plan for certain current and former executive officers, which is frozen to future accruals.
Other Postretirement Benefits ("OPEB")
In addition to providing pension benefits, we provide certain healthcare and life insurance benefits for certain domestic retired employees. We accrue the estimated cost of providing postretirement benefits during the working careers of those employees who could become eligible for such benefits when they retire. We fund these benefits as the retirees submit claims.
Retiree medical welfare changes
Under the current Hawesville labor agreement, employees who retire during the term of the labor agreement have been divided into sub-groups based on attributes such as Medicare eligibility, hire date, age and years of service. Levels of benefits are defined for the sub-groups and range from no substantive change from the benefits provided under the previous labor agreement to replacement of the defined retiree medical benefit program with individual health reimbursement accounts for each eligible participant. The health reimbursement accounts are funded based on established rates per hour worked by each eligible participant. Eligible participants will be able to withdraw from their health reimbursement accounts to fund their own retiree medical coverage.
During 2017, the Company amended its non-union retiree medical and life insurance benefits to align the Company’s benefits with the market and achieve a uniform retiree medical benefit design across the Company’s U.S. locations. Effective January 1, 2018, non-union retiree medical and life insurance benefits are restricted to current participants who meet the eligibility criteria as of January 1, 2018. Additionally, effective January 1, 2019, Century no longer administers non-union retiree medical, prescription drug, dental, or vision benefits and instead makes fixed health reimbursement account contributions.
Obligation and Funded Status
The change in benefit obligation and change in plan assets as of December 31 are as follows:
PensionOPEB
2023202220232022
Change in benefit obligation:    
Benefit obligation at beginning of year$263.0 $360.1 $73.7 $99.6 
Service cost2.4 4.3 0.1 0.2 
Interest cost14.0 10.3 3.8 2.9 
Plan amendments1.1 — — — 
Actuarial (gain) loss10.1 (87.7)4.5 (23.2)
Medicare Part D— — 0.3 0.2 
Benefits paid(18.8)(24.0)(6.7)(6.1)
Curtailment— — — 0.1 
Benefit obligation at end of year$271.8 $263.0 $75.7 $73.7 
The increases in both the defined benefit plans' and OPEB plans' benefit obligation were mainly driven by the interest cost and actuarial losses in 2023, which were primarily attributable to the increases in the discount rates from fiscal year 2022 to 2023.
PensionOPEB
2023202220232022
Change in plan assets:    
Fair value of plan assets at beginning of year$216.6 $329.7 $— $— 
Actual return on plan assets20.7 (93.3)— — 
Employer contributions6.3 4.2 6.4 5.9 
Medicare Part D subsidy received— — 0.3 0.2 
Benefits paid(18.8)(24.0)(6.7)(6.1)
Fair value of assets at end of year$224.8 $216.6 $— $— 
The change in actual return on plan assets in 2023 was primarily attributable to fluctuations in market prices during the year.
 PensionOPEB
 2023202220232022
Funded status of plans:    
Funded status$(46.9)$(46.3)$(75.7)$(73.7)
Amounts recognized in the Consolidated Balance Sheets:
Current liabilities(1.8)(1.8)(6.5)(6.1)
Non-current liabilities(45.1)(44.5)(69.2)(67.6)
Net amount recognized$(46.9)$(46.3)$(75.7)$(73.7)
Amounts recognized in accumulated other comprehensive loss (pre-tax):  
Net loss$85.2 $86.7 $14.8 $10.4 
Prior service cost (benefit)1.8 0.8 — — 
Total$87.0 $87.5 $14.8 $10.4 
Pension Plans That Are Not Fully Funded
At December 31, 2023, the projected benefit obligation, accumulated benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets were $271.8 million, $267.6 million, and $224.8 million, respectively.
At December 31, 2022, the projected benefit obligation, accumulated benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets were $263.0 million, $258.8 million and $216.6 million, respectively.
Components of net periodic benefit cost and other amounts recognized in other comprehensive loss:
Net Periodic Benefit Cost:
 Year Ended December 31,
 PensionOPEB
 202320222021202320222021
Service cost$2.4 $4.3 $4.7 $0.1 $0.2 $0.2 
Interest cost14.0 10.3 9.6 3.8 2.9 2.4 
Expected return on plan assets(15.1)(23.5)(22.4)— — — 
Amortization of prior service costs0.1 0.1 0.1 — (1.3)(3.2)
Amortization of net loss6.0 3.5 6.1 0.2 1.3 2.3 
Net periodic benefit cost7.4 (5.3)(1.9)4.1 3.1 1.7 
Curtailment benefit (1)
— — — — (8.9)— 
Total benefit cost$7.4 $(5.3)$(1.9)$4.1 $(5.8)$1.7 
(1)During 2022, we re-measured certain other postretirement benefits triggered by the Hawesville smelter curtailment, leading to a non-cash OPEB curtailment benefit totaling $8.9 million for the year ended December 31, 2022.
Other changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss (pre-tax):
 Year Ended December 31,
 PensionOPEB
 2023202220232022
Net loss (gain)$4.4 $29.0 $4.5 $(23.1)
Prior service cost (benefit)1.2 — — — 
Amortization of net loss, including recognition due to settlement(6.0)(3.5)(0.2)(1.4)
Amortization of prior service (cost) benefit, including curtailment(0.1)(0.1)— 10.3 
Total amount recognized in other comprehensive loss(0.5)25.4 4.3 (14.2)
Net periodic benefit cost7.4 (5.3)4.1 (5.8)
Total recognized in net periodic benefit cost and other comprehensive loss$6.9 $20.1 $8.4 $(20.0)
Weighted average assumptions used to determine benefit obligations at December 31:
PensionOPEB
2023202220232022
Discount rate (1)
5.19%5.50%5.19%5.57%
Rate of compensation increase (2)
3.5%
4%/3.5%
3.5%
4%/3.5%
Measurement date12/31/202312/31/202212/31/202312/31/2022
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
 PensionOPEB
 202320222021202320222021
Measurement date12/31/202212/31/202112/31/202012/31/202212/31/202112/31/2020
Fiscal year end12/31/202312/31/202212/31/202112/31/202312/31/202212/31/2021
Discount rate (1)
5.50%2.94%2.77%5.57%2.64%1.89%
Rate of compensation increase (2)
4%/3.5%
3%/3.5%
3%/3.5%
4%/3.5%
3%/3.5%
3%/3.5%
Expected return on plan assets (3)
7.25%7.25%7.25%—%—%—%
(1)We use the Ryan Above Median Yield Curve to determine the discount rate.
(2)For 2023, the rate of compensation increase is 4.0% per year for the first year and 3.5% per year thereafter. For 2022 and 2021, the rate of compensation increase is 3.0% per year for the first year and 3.5% per year thereafter.
(3)The rate for each of our defined benefit plans was selected by taking into account our expected asset mix and is based on historical performance as well as expected future rates of return on plan assets.
For measurement purposes, medical cost inflation is initially estimated to be 7.0%, and 6.5% for pre- and post-65 participants, respectively, declining to 4.5% over ten years and continuing thereafter.
Benefit Plan Assets
Pension Plan Investment Strategy and Policy
The Pension Plans’ assets are invested in a prudent manner for the exclusive purpose of providing benefits to participants.
Other objectives are to:
•Provide a total return that, over the long term, provides sufficient assets to fund the pension plan liabilities subject to a level of risk, contributions and pension expense deemed appropriate by the company.
•Minimize, where possible, pension expense volatility, and inclusion of liability driven investing as an investment strategy when appropriate. As the funding ratio improves, the objectives will evolve to minimize the funded status volatility.
•Diversify investments within asset classes to reduce the impact of losses in single investments.
The assets of the Pension Plans are invested in compliance with ERISA, as amended, and any subsequent applicable regulations and laws.
Performance
Our performance objective is to outperform the return of weighing passive investment alternatives by the policy target allocations after fees at a comparable level of risk. This investment objective is expected to be achieved over the long term and is measured over rolling multi-year periods. Peer-relative performance comparisons will also be considered especially when performance deviates meaningfully from market indexes. Investment objectives for each asset class are included below.
Asset Allocation Policy
Asset allocation policy is the principal method for achieving the Pension Plans' investment objectives stated above. The Pension Plans’ weighted average long-term strategic asset allocation policy targets are as follows:
 Pension Plan Asset Allocation
 2023 TargetDecember 31, 2023December 31, 2022
Return seeking assets:
Global equity50%44%46%
Diversified credit15%15%19%
Real assets10%10%14%
Liability hedging assets25%28%21%
Cash—%3%—%
 100%100%100%
Global equities are held for their long-term expected return premium over fixed income investments and inflation. Fixed income is held for diversification relative to equities, and as a hedging instrument to interest rate volatility for the pension obligation. Diversified Credit and Real Assets are held for diversification relative to equities and for income generation.
The strategic role of global equities is to:
•Provide higher expected returns of the major asset classes.
•Maintain a diversified exposure within global stock markets through the use of multi-manager portfolio     strategies.
The strategic role of fixed income is to:
•Diversify the Pension Plans’ equity exposure by investing in fixed income securities that exhibit a low correlation to equities, thereby lowering the overall return volatility of the entire investment portfolio.
•Maintain a diversified exposure within the U.S. fixed income market through the use of portfolio strategies targeting treasury bond exposures.
•Hedge the interest rate risk of the pension obligation by investing in securities that target a similar duration to the pension obligation cash flows.
The strategic role of diversified credit is to:
•Diversify the Pension Plans’ equity exposure by investing in alternative credit securities that exhibit a low correlation to equities, thereby lowering the overall return volatility of the entire investment portfolio.
•Maintain a diversified exposure within the alternative credit markets through the use of multi-manager portfolio strategies targeting, but not limited to, securitized credit, high yield securities, and emerging market debt.
•Achieve returns in excess of passive indexes through the use of active investment managers and strategies.
The strategic role of real assets is to:
•Diversify the Pension Plans’ equity exposure by investing in real assets that exhibit a low correlation to equities, thereby lowering the overall return volatility of the entire investment portfolio.
•Maintain a diversified exposure within the real asset markets through the use of multi-manager portfolio strategies targeting listed and unlisted exposures.
•Achieve returns in excess of passive indexes through the use of active investment managers and strategies.
The long-term strategic asset allocation policy is reviewed regularly or whenever significant changes occur to Century’s or the Pension Plans' financial position and liabilities.
Fair Value Measurements of Pension Plan assets
The following table sets forth by level the fair value hierarchy our Pension Plans' assets. These assets are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Our assessment of the significance of a particular input to the fair value measurement requires judgment, and may affect the valuation of fair value assets and liabilities and the placement within the fair value hierarchy levels.
As more fully described within Note 7. Fair Value Measurements, the Company uses a three-level fair value hierarchy that prioritizes the inputs used to measure fair values. The fair value hierarchy provides transparency regarding the inputs we use to measure fair value. We categorize each fair value measurement in its entirety into the following three levels, based on the lowest level input that is significant to the entire measurement:
•Level 1 Inputs – quoted prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity can access at the measurement date.
•Level 2 Inputs – inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.
•Level 3 Inputs – unobservable inputs for the asset or liability.
The following summarizes the Company’s Pension Plans' assets fair value by asset category:
As of December 31, 2023
Level 1Level 2Level 3Assets measured at NAVTotal
Cash and cash equivalents$— $— $— $4.6 $4.6 
Global Equity— — — 100.0 100.0 
Diversified Credit— — — 34.7 34.7 
Real Assets— — — 22.8 22.8 
Liability hedging assets— — — 62.7 62.7 
Total plan assets fair value$— $— $— $224.8 $224.8 
As of December 31, 2022
Cash and cash equivalents$— $— $— $1.3 $1.3 
Global Equity— — — 99.0 99.0 
Diversified Credit— — — 40.1 40.1 
Real Assets— — — 30.4 30.4 
Liability hedging assets— — — 45.8 45.8 
Total plan assets fair value$— $— $— $216.6 $216.6 
Our Pension Plans’ assets are held in certain commingled funds and group trusts which do not have publicly quoted prices. The fair value of the commingled funds and group trusts is based on NAV of the underlying investments. The fair value of the underlying investments held by the commingled funds, separate accounts and common collective trusts is generally based on quoted prices in active markets. Though the Company believes the methods used to estimate fair value are consistent with those used by other market participants, the use of other methods or assumptions could result in a different estimate of fair value.
Our other postretirement benefit plans are unfunded. We fund these benefits as the retirees submit claims.
Pension and OPEB Cash Flows
During 2023 and 2022, we made contributions of approximately $6.3 million and $4.2 million, respectively, to the qualified defined benefit and SERB plans we sponsor and $6.4 million and $5.9 million, respectively, to the other postretirement benefit plans.
We expect to make the following contributions for 2024:
2024
Expected pension plan contributions$4.5 
Expected OPEB benefits payments6.5 
Estimated Future Benefit Payments
The following table provides the estimated future benefit payments for the pension and other postretirement benefit plans:
 Pension BenefitsOPEB Benefits
2024$19.2 $6.5 
202519.4 6.4 
202619.5 6.4 
202719.3 6.4 
202819.5 6.2 
2029 – 203395.9 29.1 
Participation in Multi-employer Pension Plans
The union-represented employees at Hawesville are part of a United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied Industrial and Service Workers International Union ("USWA") sponsored multi-employer plan. Our contributions to the plan are determined at a fixed rate per hour worked. Currently, we do not have any plans to withdraw from or curtail participation in this plan. The risks of participating in a multi-employer plan are different from single-employer plans in the following respects:
•Assets contributed to the multi-employer plan by one employer may be used to provide benefits to employees of other participating employers.
•If a participating employer stops contributing to the plan, the unfunded obligations of the plan may be borne by the remaining participating employers.
•If a participating employer chooses to stop participating in a multi-employer plan, the employer may be required to pay the plan an amount based on the underfunded status of the plan, referred to as a withdrawal liability.
Century’s participation in the plan for the year ended December 31, 2023, is outlined in the table below.
FundSteelworkers Pension Trust
EIN / PN23-6648508 / 499
Pension Protection Act Zone Status 2022 (1)
 Green
Pension Protection Act Zone Status 2021 (1)
 Green
Subject to Financial Improvement/Rehabilitation Plan (2)
 No
Contributions of Century Aluminum 2023$0.2
Contributions of Century Aluminum 2022$1.6
Contributions of Century Aluminum 2021$1.7
Withdrawal from Plan ProbableNo
Surcharge Imposed No
Expiration Date of Collective Bargaining Agreement (2)
March 31, 2026
(1)The most recent Pension Protection Act zone status available in 2023 and 2022 is for the plan's year-end December 31, 2022 and December 31, 2021, respectively. The zone status is based on information that Century received from the plan as well as publicly available information per the Department of Labor and is certified by the plan’s actuary. Among other factors, plans in the green zone are at least 80 percent funded.
(2)The “Subject to Financial Improvement / Rehabilitation Plan” column indicates plans for which a financial improvement plan (FIP) or a rehabilitation plan (RP) is either pending or has been implemented. The last column lists the expiration date(s) of the collective-bargaining agreement(s) to which the plans are subject.
Century 401(k) Plans
We sponsor a tax-deferred savings plan under which eligible domestic employees may elect to contribute specified percentages of their compensation with Century. We match a portion of participants' contributions to the savings plan. Employee and matching contributions are considered fully vested immediately upon participation in the plan. Concurrent with the 2014 amendment to the Salaried Pension Plan that eliminated future accruals for participants who are under age 50 as of January 1, 2015 and closed the plan to new entrants, the Company increased the proportional match of contributions made to those affected by the amendment. The expense related to the plan was $5.8 million, $6.0 million, and $5.3 million for 2023, 2022, and 2021, respectively.
v3.24.0.1
Share-based Compensation
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Share-based compensation Share-based Compensation
Amended and Restated Stock Incentive Plan. Under our Amended and Restated Stock Incentive Plan (the "Stock Incentive Plan") we award service-based and performance-based share awards and nonqualified stock options to our salaried officers, non-employee directors, and other key employees. Our service-based and performance-based share awards typically vest over a period of three years from the date of grant, provided that the recipient is still our employee at the time of vesting. Our independent non-employee directors receive annual grants of service-based share awards that typically vest following 12 months of service. The Stock Incentive Plan has 12,900,000 shares authorized for issuance with approximately 3,659,885 shares remaining at December 31, 2023.
Long-Term Incentive Plan. We also grant annual long-term incentive awards under our Amended and Restated Long-Term Incentive Plan (the "LTIP"). The LTIP is designed to provide senior-level employees the opportunity to earn long-term incentive awards through the achievement of performance goals and to align compensation with the interests of our stockholders. This is achieved by linking compensation to share price appreciation and total stockholder return over a multi-year period. Awards made under the LTIP are granted subject to the Stock Incentive Plan to the extent the award is deliverable in stock. We provide two types of LTIP awards: restricted stock units ("RSU") and performance stock units ("PSU").
RSUs are stock-settled awards which do not contain any performance-based vesting requirements. PSUs can be settled in cash or stock and vest based on the achievement of pre-determined performance metrics at the discretion of the Board. Our PSU liability was approximately $3.3 million and $1.7 million as of December 31, 2023 and 2022, respectively. Both the PSUs and RSUs vest, in their entirety, after three years.
Service-based share awardsNumber
Outstanding at January 1, 2023
739,356 
Granted766,929 
Vested(293,404)
Forfeited(56,107)
Outstanding at December 31, 2023
1,156,774 
Performance-based share awardsNumber
Outstanding at January 1, 2023
444,056 
Granted407,808 
Vested(45,605)
Forfeited(203,695)
Outstanding at December 31, 2023
602,564 
 Year ended December 31,
Service-based share awards202320222021
Weighted average per share fair value of service-based share grants$12.58 $17.30 $9.97 
Fair Value Measurement of Share-Based Compensation Awards. We estimate the fair value of each stock option award using the Black-Scholes model on the date of grant. Our last grant of stock options, awarded in 2009, expired in May 2019. We have not granted any stock options since 2009. For our service-based awards, fair value is equal to the closing stock price on the date of grant. For our performance-based awards, fair value is equal to the closing stock price at each reporting period end.
The following table summarizes the compensation cost recognized for the years ended December 31, 2023, 2022 and 2021 for all service-based and performance-based share awards. The compensation cost is included as part of selling, general and administrative expenses in our Consolidated Statements of Operations.
Year ended December 31,
202320222021
Share-based compensation (benefit) expense reported:   
Performance-based share (benefit) expense$2.0 $(5.0)$12.5 
Service-based share expense4.6 4.4 8.3 
Total share-based compensation (benefit) expense before income tax6.6 (0.6)20.8 
Income tax— — — 
Total share-based compensation (benefit) expense, net of income tax$6.6 $(0.6)$20.8 
No share-based compensation cost was capitalized during these periods and there were no significant modifications of any share-based awards in 2023, 2022 and 2021. As of December 31, 2023, we had unrecognized compensation cost of $10.4 million before taxes. This cost will be recognized over a weighted average period of 1.8 years.
v3.24.0.1
Earnings Per Share
 12 Months Ended
Dec. 31, 2023
Earnings Per Share [Abstract]  
Earnings Per Share Earnings Per Share
Basic EPS amounts are calculated by dividing net income (loss) allocated to common stockholders by the weighted average number of common shares outstanding. Diluted EPS amounts assume the issuance of common stock for all potentially dilutive common shares outstanding.
The following table shows the basic and diluted earnings (loss) per share for 2023, 2022, and 2021:
For the year ended December 31, 2023
Net Loss
Shares
(in millions)
Per Share
Net loss attributable to Century stockholders$(43.1)
Amount allocated to common stockholders100 %
Basic and Diluted EPS:(1)
$(43.1)92.4 $(0.47)
For the year ended December 31, 2022
Net Loss
Shares
(in millions)
Per Share
Net loss attributable to Century stockholders$(14.1)
Amount allocated to common stockholders100 %
Basic and Diluted EPS:(1)
$(14.1)91.4 $(0.15)
For the year ended December 31, 2021
Net Loss
Shares
(in millions)
Per Share
Net loss attributable to Century stockholders$(167.1)  
Amount allocated to common stockholders100 %  
Basic and Diluted EPS:(1)
$(167.1)90.2 $(1.85)
Securities excluded from the calculation of diluted EPS (in millions)(1):
2023
2022
2021
Share-based compensation
1.0 1.7 2.7 
Convertible preferred shares5.4 5.8 6.3 
Convertible senior notes4.6 4.6 4.8 
(1)In periods when we report a net loss, all share-based compensation awards, convertible preferred shares and convertible senior notes are excluded from the calculation of diluted weighted average shares outstanding because of their anti-dilutive effect on earnings (loss) per share.
v3.24.0.1
Income Taxes
 12 Months Ended
Dec. 31, 2023
Income Tax Disclosure [Abstract]  
Income Taxes Income Taxes
The components of pre-tax book income (loss) consist of the following:
Year Ended December 31,
202320222021
U.S.$78.0 $(193.6)$(250.5)
Foreign (144.8)227.0 52.9 
Total $(66.8)$33.4 $(197.6)
Significant components of income tax expense consist of the following:
Year Ended December 31,
202320222021
Current:   
U.S. federal current expense (benefit)$0.5 $— $— 
State current expense (benefit)— 0.2 — 
Foreign current expense (benefit)15.8 4.0 0.1 
Total current expense (benefit)16.3 4.2 0.1
Deferred:   
U.S. federal deferred benefit(0.3)(0.3)(0.2)
State deferred benefit(0.1)— — 
Foreign deferred tax (benefit) expense(30.5)43.5 (30.5)
Total deferred (benefit) expense(30.9)43.2 (30.7)
Total income tax (benefit) expense$(14.6)$47.4 $(30.6)
A reconciliation of the statutory U.S. Federal income tax rate to the effective income tax rate on income (loss) is as follows:
Year Ended December 31,
 202320222021
Federal Statutory Rate21.0 %21.0 %21.0 %
Permanent differences1.1 (15.2)(0.3)
State taxes, net of Federal benefit(0.1)0.1 — 
Rate change(0.3)0.4 2.5 
Foreign earnings taxed at different rates than U.S.2.0 (0.8)(3.9)
Valuation allowance3.6 (4.2)(15.9)
Helguvik investment— — 26.4 
Foreign dividends and inclusions(12.7)122.9 (10.1)
Net operating loss expiration and remeasurement(8.0)43.1 (5.2)
Filing differences0.6 (19.1)(0.1)
Changes in uncertain tax reserves(1.3)(5.3)1.3 
Advanced Manufacturing Production Credit18.6 — — 
Other(2.7)(0.9)(0.2)
Effective tax rate21.8 %142.0 %15.5 %
The effective tax rate for the year ending December 31, 2023 was 21.8% compared to the statutory US tax rate of 21%. The increase is primarily a result of the calculated foreign inclusions, partially offset by the non-taxable benefit of the Advanced Manufacturing Credit under Section 45X discussed below.
In August 2022, President Biden signed the IRA into law. The IRA provides several tax incentives to promote clean energy and the production of critical minerals in the U.S., including a refundable tax credit, pursuant to Section 45X of the Internal Revenue Code. Tax credits, such as refundable credits whose realization does not depend on the entity’s generation of taxable income like the refundable tax credit provided by the IRA are not considered an element of income tax accounting under ASC 740. After considering US GAAP, the Company has concluded it is appropriate to apply IAS 20, Accounting for Government Grants and Disclosure of Government Assistance, to account for the refundable tax credit as an income grant.
Section 45X of the IRA contains a production tax credit equal to 10% of certain eligible production costs, including, without limitation, labor, energy, depreciation and amortization and overhead expenses. On December 14, 2023, the U.S. Department of the Treasury and the Internal Revenue Service released proposed rules to provide guidance on the production tax
credit requirements under Internal Revenue Code Section 45X (the “Proposed Regulations”). The Proposed Regulations provide guidance on rules that taxpayers must satisfy to qualify for the Section 45X tax credit. For the year ended December 31, 2023, we recognized $56.5 million as a reduction in Cost of goods sold and $2.8 million as a reduction in Selling, general and administrative expenses on the Consolidated Statements of Operations and recorded an equal amount as a Manufacturing credit receivable on the Consolidated Balance Sheets.

In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures ("ASU 2023-09"). ASU 2023-09 enhanced annual disclosures regarding the rate reconciliation and income taxes paid information. For public business entities, ASU 2023-09 is effective for annual periods beginning after December 15, 2024. The Company is evaluating the impact of this guidance on its consolidated financial statements and related disclosures.

The Company’s accounting policy with respect to releasing income tax effects from accumulated other comprehensive income is to apply a security by security approach whereby the tax effects are measured based on the change in the unrealized gains or losses reflected in other comprehensive loss.
Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes.
The significant components of our deferred tax assets and liabilities as of December 31 are as follows:
20232022
Deferred tax assets:  
Accrued postretirement benefit cost$30.7 $25.7 
Net operating losses 467.6 395.8 
Disallowed interest expense29.2 27.7 
Derivative and hedging contracts1.2 — 
Fixed asset tax over book basis9.2 17.0 
Other28.3 26.1 
Total deferred tax assets566.2 492.3 
Valuation allowance(537.6)(487.9)
Net deferred tax assets$28.6 $4.4 
Deferred tax liabilities:  
Fixed asset book over tax basis(62.0)(60.5)
Derivatives— (19.0)
Foreign basis differences(18.1)(19.8)
Other(20.6)(7.9)
Total deferred tax liabilities(100.7)(107.2)
Net deferred tax liability$(72.1)$(102.8)
We regularly assess the likelihood that deferred tax assets will be recovered from future taxable income. To the extent we believe that it is more likely than not that a deferred tax asset will not be realized, a valuation allowance is established. When a valuation allowance is established or increased, an income tax charge is included in the consolidated statement of operations and net deferred tax assets are adjusted accordingly. Future changes in tax laws, statutory tax rates and taxable income levels could result in actual realization of the deferred tax assets being materially different from the amounts provided for in the consolidated financial statements. If the actual recovery amount of the deferred tax asset is less than anticipated, we would be required to write-off the remaining deferred tax asset and increase the tax provision.
We have a valuation allowance of $537.6 million recorded against our net U.S. and Jamaican deferred tax assets, and a portion of our Icelandic deferred tax assets as of December 31, 2023. The Company is subject to the provisions of ASC 740-10, Income Taxes, which requires that the effect on deferred tax assets and liabilities of a change in tax rates be recognized in the period the tax rate change was enacted. The change in net operating losses, fixed asset book over tax basis, and the related valuation allowances, were the result of purchase accounting adjustments related to the acquisition of Jamalco, which was offset by a decrease in deferred tax liabilities primarily related to the Company's hedging derivatives.
The changes in the valuation allowance are as follows:
Year Ended December 31,
202320222021
Beginning balance, valuation allowance$487.9 $485.8 $499.4 
Remeasurement of deferred tax assets— — — 
Release of valuation allowance— — — 
Expiration of net operating losses(7.2)(15.4)(13.2)
Other change in valuation allowance56.9 17.5 (0.4)
Ending balance, valuation allowance$537.6 $487.9 $485.8 
The significant components of our NOLs are as follows:
20232022
Federal (1)
$1,533.5 $1,487.8 
State (2)
1,221.0 1,182.7 
Foreign (3)(4)
344.4 106.0 
(1)US federal NOLs begin to expire in 2028.
(2)US state NOLs begin to expire in 2027.
(3)NOLs in Iceland expire between 2024 and 2026.
(4)NOLs in Jamaica do not expire.
Our ability to utilize our deferred tax assets to offset future federal taxable income may be significantly limited if we experience an "ownership change" as defined in the Code. In general, an ownership change would occur if our "five-percent shareholders," as defined under the Code, collectively increase their ownership in us by more than 50 percentage points over a rolling three-year period. Future transactions in our stock that may not be in our control may cause us to experience such an ownership change and thus limit our ability to utilize net operating losses, tax credits and other tax assets to offset future taxable income.
A reconciliation of the beginning and ending amounts of gross unrecognized tax benefits (excluding interest) is as follows:
202320222021
Balance as of January 1,  $2.2 $4.0 $6.5 
Additions based on tax positions related to the current year1.3 0.3 — 
Decreases due to lapse of applicable statute of limitations(0.5)(2.1)(2.5)
Settlements — — — 
Balance as of December 31,$3.0 $2.2 $4.0 
As of December 31, 2023, the Company’s gross unrecognized tax benefits totaled $3.0 million. After considering the deferred tax accounting impact, it is expected that about $1.4 million of the total as of December 31, 2023 would favorably affect the effective tax rate if resolved in the Company’s favor. Included in the above balances are tax positions relating to temporary differences where there is uncertainty about the timing of tax return inclusion, but not that the amounts will ultimately be tax deductible. Because of the impact of deferred tax accounting, other than interest and penalties, the timing would not impact the annual effective tax rate but could accelerate the payment of cash to the taxing authority to an earlier period. We do not expect a significant change in the balance of unrecognized tax benefits within the next twelve months. It is our policy to recognize potential accrued interest and penalties related to unrecognized tax benefits in income tax expense.
Century and its subsidiaries file income tax returns in the U.S. federal jurisdiction, various state and local jurisdictions, and several foreign jurisdictions.
Our federal income tax returns have been reviewed by the IRS through 2010. However, we have NOLs beginning in 2008 that are available for carryforward to future years. Under U.S. tax law, NOLs may be adjusted by the IRS until the statute of limitations expires for the year in which the NOL is used. Accordingly, our 2008 and later NOLs may be reviewed until they are used or expire.
We are subject to examination by tax authorities according to statutory periods defined in each jurisdiction. The earliest statutory period open is beginning in 2018.
v3.24.0.1
Commitments and Contingencies
 12 Months Ended
Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]  
Commitments and Contingencies Commitments and Contingencies
We have pending against us or may be subject to various lawsuits, claims and proceedings related primarily to employment, commercial, stockholder, environmental, safety and health matters and are involved in other matters that may give rise to contingent liabilities. While the results of such matters and claims cannot be predicted with certainty, we believe that the ultimate outcome of any such matters and claims will not have a material adverse impact on our financial condition, results of operations or liquidity. However, because of the nature and inherent uncertainties of litigation and estimating liabilities, should the resolution or outcome of these actions be unfavorable, our business, financial condition, results of operations and liquidity could be materially and adversely affected.
In evaluating whether to accrue for losses associated with legal or environmental contingencies, it is our policy to take into consideration factors such as the facts and circumstances asserted, our historical experience with contingencies of a similar nature, the likelihood of our prevailing and the severity of any potential loss. For some matters, no accrual is established because we have assessed our risk of loss to be remote. Where the risk of loss is probable and the amount of the loss can be reasonably estimated, we record an accrual, either on an individual basis or with respect to a group of matters involving similar claims, based on the factors set forth above. While we regularly review the status of, and our estimates of potential liability associated with, contingencies to determine the adequacy of any associated accruals and related disclosures, the ultimate amount of loss may differ from our estimates.
Legal Contingencies
Ravenswood Retiree Medical Benefits changes
In November 2009, Century Aluminum of West Virginia ("CAWV") filed a class action complaint for declaratory judgment against the United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied Industrial and Service Workers International Union ("USW"), the USW’s local and certain CAWV retirees, individually and as class representatives ("CAWV Retirees"), seeking a declaration of CAWV’s rights to modify/terminate retiree medical benefits. Later in November 2009, the USW and representatives of a retiree class filed a separate suit against CAWV, Century Aluminum Company, Century Aluminum Master Welfare Benefit Plan, and various John Does with respect to the foregoing. On August 18, 2017, the District Court for the Southern District of West Virginia approved a settlement agreement in respect of these actions, pursuant to which agreement, CAWV agreed to make payments into a trust for the benefit of the CAWV Retirees in the aggregate amount of $23.0 million over the course of 10 years. Upon approval of the settlement, we paid $5.0 million to the aforementioned trust in September 2017 and recognized a gain of $5.5 million to arrive at the-then net present value of $12.5 million. CAWV has agreed to pay the remaining amounts under the settlement agreement in annual increments of $2.0 million for nine years. As of December 31, 2023, $2.0 million was recorded in other current liabilities and $3.3 million was recorded in other liabilities.
PBGC Settlement
In 2013, we entered into a settlement agreement with the Pension Benefit Guarantee Corporation ("the PBGC") regarding an alleged "cessation of operations" at our Ravenswood facility (the "PBGC Settlement Agreement"). Pursuant to the terms of the PBGC Settlement Agreement, we agreed to make additional contributions (above any minimum required contributions) to our defined benefit pension plans totaling approximately $17.4 million. Under certain circumstances, in periods of lower primary aluminum prices relative to our cost of operations, we were able to defer one or more of these payments, provided that we provide the PBGC with acceptable security for such deferred payments. We did not make any contributions for the three or nine month periods ended September 30, 2021, and 2020. We historically elected to defer certain payments under the PBGC Settlement Agreement and provided the PBGC with the appropriate security. On October 1, 2021, we amended the PBGC Settlement Agreement (the "Amended PBGC Settlement Agreement") such that we removed the deferral mechanism and agreed to contribute approximately $2.4 million per year to our defined benefit pension plans for a total of approximately $9.6 million, over four years beginning on November 30, 2022 and ending on November 30, 2025, subject to acceleration if
certain terms and conditions are met in such amendment. As of December 31, 2023, we have made contributions of $6.9 million related to the Amended PBGC Settlement Agreement.
Power Commitments and Contingencies
Hawesville
Hawesville has a power supply arrangement with Kenergy and Century Marketer, LLC (“Century Marketer"), Century's wholly-owned subsidiary that acts as a MISO market participant. Under this arrangement, Hawesville gets access to power at Midcontinent Independent System Operator ("MISO") pricing plus transmission and other costs. As the MISO Market Participant, Century Marketer purchases power from MISO for resale to Kenergy, which then resells the power to Hawesville. The power supply arrangement with Kenergy has an effective term through May 2028.
Sebree
Sebree has a power supply arrangement with Kenergy and Century Marketer, LLC (“Century Marketer"), Century's wholly-owned subsidiary that acts as a MISO market participant. Under this arrangement, Sebree gets access to power at Midcontinent Independent System Operator ("MISO") pricing plus transmission and other costs. As the MISO Market Participant, Century Marketer purchases power from MISO for resale to Kenergy, which then resells the power to Sebree. The power supply arrangement with Kenergy has an effective term through May 2028.
Mt. Holly
CASC has a power supply agreement with Santee Cooper that has an effective term from January 1, 2024 and runs through December 2026. Under this power supply agreement, 100% of Mt. Holly’s electrical power requirements are supplied from Santee Cooper’s generation at cost of service based rates.
Grundartangi
Grundartangi has power purchase agreements for approximately 545 MW with HS Orka hf ("HS"), Landsvirkjun and Orkuveita Reykjavikur ("OR") to provide power to its Grundartangi smelter. These power purchase agreements expire on various dates from 2026 through 2036 (subject to extension). The power purchase agreements with HS and OR provide power at LME-based variable rates for the duration of these agreements. In July 2021, Grundartangi reached an agreement with Landsvirkjun for an extension of its existing 161 MW power contract that would have expired in December 2023. Under the terms of the extension, Landsvirkjun will continue to supply power to Grundartangi from January 1, 2024 through December 31, 2026 and will increase the existing contract from 161 MW to 182 MW over time to provide the necessary flexibility to support the most recent capacity creep requirements and future growth opportunities for value-added products at the Grundartangi plant, including the casthouse project. In September 2022, this agreement was amended to provide for 42 MW at a fixed price and 119 MW at rates linked to Nord Pool plus transmission through 2023 and beginning January 1, 2024 through December 31, 2026, this agreement allows for fixed rates plus a small variable rate portion of the full 182 MW. Grundartangi also has a 25 MW power purchase agreement with Landsvirkjun at LME-based variable rates.
Other Commitments and Contingencies
Contingent Obligation
We have a contingent obligation in connection with the “unwind” of a contractual arrangement between CAKY, Big Rivers Electric Corporation ("Big Rivers") and a third party and the execution of a long-term cost-based power contract with Kenergy, a member of a cooperative of Big Rivers, in July 2009. This contingent obligation consists of the aggregate payments made to Big Rivers by the third party on CAKY’s behalf in excess of the agreed upon base amount under the long-term cost-based power contract with Kenergy. Our obligation to make repayments is contingent upon certain operating criteria for Hawesville and the LME price of primary aluminum. When the conditions for repayment are met, and for so long as those conditions continue to be met, we will be obligated to make principal and interest payments, in up to 72 monthly payments. Interest accrues at an annual rate equal to 10.94% and the term of the agreement is through December 2028.
Based on the LME forward market prices for primary aluminum at December 31, 2023, and current level of Hawesville's operations, including the temporary curtailment, we believe that we will not be required to make payments on the contingent obligation during the term of the agreement, which expires in 2028. We recognized a derivative asset which offsets our
contingent obligation. As a result, our net liability decreased and we recognized a gain of $1.4 million for each of the years ended December 31, 2023 and 2022. These amounts are exactly offset by interest expense on the contingent obligation which is recorded in interest expense. Future increases in the LME forward market and increased operations at Hawesville may result in a partial or full derecognition of the derivative asset and a corresponding recognition of a loss.
The following table provides information about the balance sheet location and gross amounts offset:
December 31,
Offsetting of financial instruments and derivativesBalance sheet location20232022
Contingent obligation – principalOther liabilities$(12.9)$(12.9)
Contingent obligation – accrued interestOther liabilities(18.0)(16.6)
Contingent obligation – derivative assetOther liabilities30.9 29.5 
$— $— 
Labor Commitments
The bargaining unit employees at our Grundartangi, Vlissingen, Hawesville, Sebree and Jamalco facilities are represented by labor unions, representing approximately 60% of our total workforce.
Approximately 87% of Grundartangi’s work force is represented by five labor unions, governed by a labor agreement that establishes wages and work rules for covered employees. This agreement is effective through December 31, 2024.
100% of Vlissingen’s workforce is represented by the Federation for the Metal and Electrical Industry ("FME"), a Netherlands' employers' organization for companies in the metal, electronics, electrical engineering and plastic sectors. The FME negotiates working conditions with trade unions on behalf of its members, which, when agreed upon, are then applicable to all employees of Vlissingen. The current labor agreement is effective through May 31, 2024.
Approximately 42% of our U.S. based work force is represented by USW through separately negotiated labor agreements for each facility. The labor agreement for Hawesville employees is effective through April 1, 2026. Upon announcement of the temporary curtailment, Hawesville and the USW local union entered into effects bargaining. An agreement was reached on July 19, 2022, covering the curtailment period. Century Sebree's labor agreement with USW for its employees is effective through October 28, 2028. Mt. Holly employees are not represented by a labor union.
Approximately 62% of Jamalco's work force is represented by the Union of Technical, Administrative, and Supervisory Personnel ("UTASP") through separately labor agreements for hourly and salaried employee groups. Both contracts are effective through December 31, 2023. Jamalco is currently in the process of negotiating new contracts with both the salaried and hourly employee groups.
v3.24.0.1
Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2023
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Asset Retirement Obligations
The reconciliation of the changes in our AROs is presented below:
Year ended December 31,
20232022
Beginning balance$21.2 $20.7 
Additional ARO liabilities incurred3.1 3.8 
ARO liabilities settled(1.5)(4.6)
Accretion expense2.0 1.9 
Acquired ARO liabilities (See Note 2)23.9 — 
Revisions in estimated cash flows2.4 (0.6)
Ending balance$51.1 $21.2 
Current portion of asset retirement obligations (1)
1.6 1.6 
Asset retirement obligations - less current portion$49.5 $19.6 
v3.24.0.1
Business Segments
 12 Months Ended
Dec. 31, 2023
Segment Reporting [Abstract]  
Business Segments Business Segments
Century Aluminum is a producer of primary aluminum, which trades as a global commodity, and owns a 55% interest in a bauxite mining and alumina refinery joint venture. We are organized as a holding company with each of our operating primary aluminum smelters managed and operated as a separate facility reporting to our corporate headquarters. Each of our operating primary aluminum smelters and our bauxite and alumina refinery meets the definition of an operating segment. We evaluated the similar economic and other characteristics, including nearly identical products, production processes, customers and distribution and have aggregated our five operating segments into one reportable segment, primary aluminum, based on these factors. In addition, all of our operating segments share several key economic factors inherent in their common products and production processes. For example, all of our facilities' revenue is based on market pricing. Our facilities have a similar customer base and utilize similar distribution methods to ship products.
A reconciliation of our consolidated assets to the total of primary aluminum segment assets is provided below.
Segment assets (1)
202320222021
Primary$1,808.1 $1,432.4 $1,513.3 
Corporate, unallocated38.4 39.6 56.6 
Total assets$1,846.5 $1,472.0 $1,569.9 
(1)Segment assets include accounts receivable, due from affiliates, prepaid and other current assets, leases - right of use assets, inventory, intangible assets and property, plant and equipment, net; the remaining assets are unallocated corporate assets.
Geographic information
Our net sales are attributed to geographic area based on the location of the selling operation. Included in the consolidated financial statements are the following amounts related to geographic locations:
 202320222021
Net sales: (1)
   
United States$1,358.6 $1,737.2 $1,413.0 
Iceland826.8 1,040.1 799.5 
Long-lived assets: (2)
   
United States$219.1 $244.9 $400.1 
Iceland529.4 491.0 490.1 
Jamaica275.8 — — 
Other55.1 58.3 61.5 
(1)Includes sales of primary aluminum, scrap aluminum and alumina, and purchased aluminum and alumina.
(2)Includes long-lived assets other than financial instruments and deferred taxes.
Major customer information
Revenues from one customer in 2023 and two customers in 2022 and 2021 exceeded 10% of our net sales . A loss of these customers could have a material adverse effect on our results of operations. The net sales related to the customers is as follows:
Year Ended December 31,
202320222021
Glencore$1,612.1 $1,671.1 $1,337.0 
Southwire— 331.3 304.6 
v3.24.0.1
Derivatives
 12 Months Ended
Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Derivatives Derivatives
As of December 31, 2023, we had an open position of 36,633 tonnes related to LME forward financial sales contracts to fix the forward LME aluminum price. These contracts are expected to settle monthly through December 2024. We have also entered into financial contracts with various counterparties to offset fixed price sales arrangements with certain of our customers ("fixed for floating swaps") to remain exposed to the LME and MWP aluminum prices. As of December 31, 2023, we had 318 tonnes related to fixed for floating swaps that will settle monthly through February 2024.
We previously entered into financial contracts to hedge a portion of Grundartangi's exposure to the Nord Pool power market (“Nord Pool power price swaps”). The Nord Pool power price swaps are settled in Euros; therefore, we entered into financial contracts to hedge the risk of fluctuations associated with the Euro ("FX swaps"). During the third quarter of 2022, we entered certain floating Nord Pool financial contracts to unwind a portion of our fixed contract position, making us predominantly hedged against Nord Pool power price fluctuations during 2023. As of December 31, 2023, we had no open Nord Pool power price swaps.
We previously entered into financial contracts to fix a portion of our exposure to the Indiana Hub power market at our Kentucky plants ("Indiana Hub power price swaps"). As of December 31, 2023, we had no open Indiana Hub power price swaps.
We have entered into forward contracts to hedge the risk of fluctuations associated with the Icelandic Krona (ISK) and Euro for contracts related to the construction of the Grundartangi casthouse and the Sebree casthouse project denominated in these currencies ("casthouse currency hedges"). As of December 31, 2023, we had an open position related to the ISK casthouse swaps of kr397.0 million and an open position related to the Euro casthouse swaps of €1.8 million that will settle through January 2024.
We previously entered into financial contracts to hedge a portion of our exposure at our operations to the NYMEX Henry Hub (“NYMEX Henry Hub natural gas price swaps”). The natural gas volume is measured per million British Thermal Units ("MMBtu"). As of December 31, 2023, we had no open NYMEX Henry Hub natural gas price swaps.
We have entered into financial contracts to hedge a portion of our exposure at our operations to Heavy Fuel Oil (“HFO price swaps”). The HFO volume is measured per barrel. As of December 31, 2023, we had an open position of 180,000 barrels. The HFO price swaps are expected to settle monthly through March 2024.
Our agreements with derivative counterparties contain certain provisions requiring collateral to be posted in the event the market value of our position exceeds the margin threshold limit of our master agreement with the counterparty. As of December 31, 2023 and December 31, 2022, we had no recorded restricted cash as collateral related to open derivative contracts under the master arrangements with our counterparties.
The following table sets forth the Company's derivative assets and liabilities that were accounted for at fair value and not designated as cash flow hedges as of December 31, 2023 and 2022, respectively:
Asset Fair Value
20232022
Commodity contracts (1)
$2.9 $129.1 
Foreign exchange contracts (2)
— — 
Total$2.9 $129.1 
 Liability Fair Value
20232022
Commodity contracts (1)
7.8 23.7 
Foreign exchange contracts (2)
0.1 7.3 
Total$7.9 $31.0 
(1)Commodity contracts reflect our outstanding LME forward financial sales contracts, fixed for floating swaps, Nord Pool power price swaps, HFO price swaps and Indiana Hub power price swaps. At December 31, 2023, $6.4 million of Due to affiliates was related to commodity contract liabilities with Glencore. At December 31, 2022, $11.9 million of Due to affiliates, and $8.3 million of Due to affiliates - less current portion was related to commodity contract liabilities with Glencore.
(2)Foreign exchange contracts reflect our outstanding FX swaps and casthouse currency hedges.
The following table summarizes the net gain (loss) on forward and derivative contracts for the years ended December 31, 2023, 2022, and 2021:
Year Ended December 31,
202320222021
Commodity contracts (1)
$63.5 $206.6 $(208.0)
Foreign exchange contracts(1.7)(9.4)(4.4)
   Total$61.8 $197.2 $(212.4)
(1)For the years ended December 31, 2023, 2022, and 2021, $0.6 million, $(13.3) million, and $116.9 million of net gain (loss), respectively, was with Glencore.
v3.24.0.1
Pay vs Performance Disclosure - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pay vs Performance Disclosure      
Net loss attributable to Century stockholders $ (43.1) $ (14.1) $ (167.1)
v3.24.0.1
Insider Trading Arrangements
 3 Months Ended
Dec. 31, 2023
Trading Arrangements, by Individual  
Rule 10b5-1 Arrangement Adopted true
Non-Rule 10b5-1 Arrangement Adopted false
Rule 10b5-1 Arrangement Terminated false
Non-Rule 10b5-1 Arrangement Terminated false
v3.24.0.1
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.
v3.24.0.1
Acquisition of Jamalco (Tables)
 12 Months Ended
Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]  
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed
The following table summarizes the estimated fair value of identified assets acquired, liabilities assumed and noncontrolling interest at the date of acquisition:
Preliminary purchase price allocationAmount
Identifiable assets acquired and liabilities assumed
Cash and cash equivalents$19.4 
Restricted cash8.3 
Accounts receivable - net7.7 
Non-trade receivables40.4 
Inventories103.9 
Prepaid and other current assets4.2 
Property, plant and equipment246.2 
Other assets26.0 
Accounts payable, trade(94.8)
Accrued and other current liabilities(30.8)
Other liabilities(35.5)
Asset retirement obligations(23.9)
Total identifiable assets acquired and liabilities assumed271.1 
Less: noncontrolling interest (2.3)
Deferred credit - preliminary bargain purchase gain273.4 
Business Acquisition, Pro Forma Information
The following unaudited pro forma financial information reflects the results of operations of the Company for the twelve months ended December 31, 2023 and 2022, respectively, as if the acquisition of Jamalco had been completed on January 1, 2022. This unaudited pro forma financial information has been prepared for informational purposes and is not necessarily indicative of the actual consolidated results of operations had the acquisition been completed on January 1, 2022, nor is the information indicative of future results of operations of the combined companies.
Year ended December 31,
20232022
Revenue$2,235.1 $2,831.0 
Earnings$(45.9)$(33.1)
v3.24.0.1
Related Party Transactions (Tables)
 12 Months Ended
Dec. 31, 2023
Related Party Transactions [Abstract]  
Schedule of related party transactions
A summary of the aforementioned significant related party sales and purchases for the years ended December 31, 2023, 2022 and 2021 is as follows:
 Year Ended December 31,
 202320222021
Net sales to Glencore$1,612.1 $1,671.1 $1,337.0 
Purchases from Glencore (1)
181.4 284.7 334.6 
(1)Includes settlements of financial contract positions.
v3.24.0.1
Revenue (Table)
 12 Months Ended
Dec. 31, 2023
Revenue from Contract with Customer [Abstract]  
Disaggregation of revenue
We disaggregate our revenue by geographical region as follows:
Year ended December 31,
Net Sales202320222021
United States$1,358.6 $1,737.2 $1,413.0 
Iceland826.8 1,040.1 799.5 
Total$2,185.4 $2,777.3 $2,212.5 
v3.24.0.1
Leases (Tables)
 12 Months Ended
Dec. 31, 2023
Leases [Abstract]  
Schedule of assets and liabilities, lessee
Our ROUA and ROUL balances for the years ended December 31, 2023 and December 31, 2022 were as follows (in millions):
December 31,
20232022
ROUA(1)
$24.7 $20.9 
ROUL - current(2)
$2.3 $1.8 
ROUL - non-current(3)
21.9 20.9 
Total ROUL$24.2 $22.7 
(1)ROUA was recorded as part of Other Assets within Non-current assets at December 31, 2023 and 2022.
(2)ROUL - current was recorded as part of Accrued and other current liabilities within Current liabilities at December 31, 2023 and 2022.
(3)ROUL - non-current was recorded as part of Other liabilities within Non-current liabilities at December 31, 2023 and 2022.
Schedule of maturities of operating lease liability balances
The undiscounted maturities of our operating lease liability balances as of December 31, 2023 are as follows (in millions):
YearDecember 31,
2024$3.9 
2025$3.7 
2026$3.3 
2027$2.9 
2028$2.9 
Thereafter$19.4 
Total $36.1 
Less: Interest$(11.9)
ROUL$24.2 
Schedule of lease, expense
Total operating expense includes the following (in millions):
December 31,
20232022
Operating leases expense$4.9 $4.5 
Short term lease expense0.6 0.4 
Total(1)
$5.5 $4.9 
(1)Total lease expense is included in cost of goods sold and selling, general, and administrative expenses on the Consolidated Statements of Operations.
v3.24.0.1
Fair Value Measurements (Tables)
 12 Months Ended
Dec. 31, 2023
Fair Value Disclosures [Abstract]  
Schedule of financial assets and liabilities at fair value on a recurring basis
Recurring Fair Value Measurements
As of December 31, 2023
Level 1Level 2Level 3Total
ASSETS:    
Cash equivalents$16.8 $— $— $16.8 
Trust assets (1)
0.2— — 0.2
Derivative instruments— 2.9 — 2.9 
TOTAL$17.0 $2.9 $— $19.9 
LIABILITIES:    
Derivative instruments— 7.9 — 7.9 
TOTAL$— $7.9 $— $7.9 
Recurring Fair Value Measurements
As of December 31, 2022
Level 1Level 2Level 3Total
ASSETS:    
Cash equivalents$5.6 $— $— $5.6 
Trust assets (1)
0.1— — 0.1 
Derivative instruments— 127.3 1.8 129.1 
TOTAL$5.7 $127.3 $1.8 $134.8 
LIABILITIES:    
Contingent obligation – net (2)
$— $— $— $— 
Derivative instruments— 26.4 4.6 31.0 
TOTAL$— $26.4 $4.6 $31.0 
(1)Trust assets are currently invested in money market funds. These trust assets are held to fund the non-qualified supplemental executive pension benefit obligations for certain of our officers.
(2)See Note 17. Commitments and Contingencies for additional information about the contingent obligation.
Schedule of valuation methodology for assets and liabilities at fair value
The following section describes the valuation techniques and inputs used for fair value measurements categorized within Level 2 or Level 3 of the fair value hierarchy:
Level 2 Fair Value Measurements:
Asset / LiabilityValuation TechniquesInputs
LME forward financial sales contractsDiscounted cash flowsQuoted LME forward market
Fixed for floating swapsDiscounted cash flowsQuoted LME forward market, quoted MWP forward market
Nord Pool power price swaps Discounted cash flowsQuoted Nord Pool forward market
Indiana Hub power price swapsDiscounted cash flowsQuoted Indiana Hub forward market
FX swaps Discounted cash flowsEuro/USD forward exchange rate
Casthouse currency hedgesDiscounted cash flowsEuro/USD forward exchange rate; ISK/USD forward exchange rate
Heavy Fuel Oil ("HFO") price swapsDiscounted cash flowsQuoted HFO forward market
The following table presents the inputs for fair value measurements categorized within Level 3 of the fair value hierarchy, along with information regarding significant unobservable inputs used to value Level 3 assets and liabilities:






Level 3 Fair Value Measurements:
As of December 31, 2023
As of December 31, 2022
Asset / LiabilityValuation TechniqueObservable InputsSignificant Unobservable InputFair Value Value/Range of Unobservable InputFair ValueValue/Range of Unobservable Input
LME forward financial sales contractsDiscounted cash flowsQuoted LME forward market
Discount rate net (1)
$— 8.58%$(2.8)8.58%
(1) Represents risk adjusted discount rate
Schedule of fair value reconciliation of Level 3 assets and liabilities measured at fair value
The following table presents the fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis.
For the twelve months ended December 31, 2023
Level 3 AssetsLevel 3 Liabilities
LME forward financial sales contractsLME forward financial sales contracts
Balance as of January 1, 2023
$1.8 $(4.6)
Transfers out of Level 3 (1)
(1.8)4.6 
Balance as of December 31, 2023
$— $— 
Change in unrealized gains (losses) (2)
$— $— 
(1)Transfer out of Level 3 due to period of time remaining in derivative contract.
(2)Gains and losses are presented in the Consolidated Statement of Operations within the line item "Net gain (loss) on forward and derivative contracts."
v3.24.0.1
Debt (Tables)
 12 Months Ended
Dec. 31, 2023
Debt Disclosure [Abstract]  
Schedule of debt
 December 31,
 20232022
Debt classified as current liabilities:  
Hancock County industrial revenue bonds ("IRBs") due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) (1)
$7.8 $7.8 
U.S. Revolving Credit Facility (2)
23.7 90.0 
Iceland Revolving Credit Facility (3)
— 35.0 
Grundartangi Casthouse Facility (4)
5.5 — 
Iceland Term Facility (5)
1.3 13.3 
Vlissingen Facility Agreement (6)
10.0 — 
Debt classified as non-current liabilities:  
Grundartangi casthouse facility, net of financing fees of $0.7 million at December 31, 2023 (4)
98.8 49.4 
Iceland Term Facility, net of financing fees of $0.0 million and current portion at December 31, 2023 (5)
— 1.2 
7.5% senior secured notes due April 1, 2028, net of financing fees of $2.6 million at December 31, 2023, interest payable semiannually
247.4 246.6 
2.75% convertible senior notes due May 1, 2028, net of financing fees of $1.5 million at December 31, 2023, interest payable semiannually
84.7 84.4 
Total$479.2 $527.7 
(1)The IRBs are classified as current liabilities because they are remarketed weekly and could be required to be repaid upon demand if there is a failed remarketing. The interest rate at December 31, 2023 was 4.00%.
(2)We incur interest at a base rate plus applicable margin as defined within the agreement. The interest rate at December 31, 2023 was 9.25%.
(3)We incur interest at a base rate plus applicable margin as defined within the agreement. The were no outstanding borrowings at December 31, 2023.
(4)We incur interest at a base rate plus applicable margin as defined within the agreement. The interest rate at December 31, 2023 was 8.86%.
(5)We incur interest at a rate equal to 3.2% plus EUR EURIBOR 1 month as published by the European Money Markets Institute as defined within the agreement. The interest rate at December 31, 2023 was 7.05%.
(6)We incur interest at a fixed rate of 8.75%.
Schedule of debt redemption rights
Redemption Rights. Prior to April 1, 2024, we may redeem the 2028 Notes, in whole or in part, at a redemption price equal to 100.00% of the principal amount plus a make-whole premium and accrued and unpaid interest, and if redeemed during the twelve-month period beginning on April 1 of the years indicated below, at the following redemption prices plus accrued and unpaid interest:
YearPercentage
2024103.750%
2025101.875%
2026 and Thereafter100.000%
Schedule of line of credit facilities
Status of our U.S. revolving credit facility:
December 31, 2023
Credit facility maximum amount$250.0 
Borrowing availability128.8 
Outstanding letters of credit issued61.4 
Outstanding borrowings23.7 
Borrowing availability, net of outstanding letters of credit and borrowings43.7 
Status of our Iceland revolving credit facility:
December 31, 2023
Credit facility maximum amount$100.0 
Borrowing availability100.0 
Outstanding letters of credit issued— 
Outstanding borrowings— 
Borrowing availability, net of outstanding letters of credit and borrowings100.0 
v3.24.0.1
Inventories (Tables)
 12 Months Ended
Dec. 31, 2023
Inventory, Net [Abstract]  
Schedule of Inventories
Inventories, at December 31, consist of the following:
 20232022
Raw materials$162.5 $64.9 
Work-in-process42.9 46.0 
Finished goods46.3 58.0 
Operating and other supplies225.3 229.9 
Inventories$477.0 $398.8 
v3.24.0.1
Property, Plant and Equipment (Tables)
 12 Months Ended
Dec. 31, 2023
Property, Plant and Equipment [Abstract]  
Summary of property, plant and equipment
Property, plant and equipment, at December 31, consist of the following:
 20232022
Land and improvements$105.1 $39.8 
Mineral Reserves57.6 — 
Buildings and improvements328.8 305.7 
Machinery and equipment1,580.7 1,495.3 
Construction in progress160.3 59.0 
 2,232.5 1,899.8 
Less accumulated depreciation, amortization and depletion(1,228.3)(1,155.4)
Property, plant and equipment - net$1,004.2 $744.4 
v3.24.0.1
Accumulated Other Comprehensive Loss (AOCL) (Tables)
 12 Months Ended
Dec. 31, 2023
Additional financial information disclosures [Abstract]  
Components of Accumulated Other Comprehensive Loss
Components of AOCL20232022
Defined benefit plan liabilities$(101.8)$(98.0)
Unrealized gain on financial instruments1.6 1.7 
Other comprehensive loss before income tax effect(100.2)(96.3)
Income tax effect(1)
2.3 2.3 
Accumulated other comprehensive loss$(97.9)$(94.0)
(1)The allocation of the income tax effect to the components of other comprehensive loss is as follows:
20232022
Defined benefit plan liabilities$2.6 $2.6 
Unrealized loss on financial instruments(0.3)(0.3)
The following table summarizes the changes in the accumulated balances for each component of AOCL:
Defined benefit plan and other postretirement liabilitiesUnrealized gain (loss) on financial instrumentsTotal, net of tax
Balance, December 31, 2020
$(120.6)$1.8 $(118.8)
Other comprehensive income (loss) before reclassifications31.6 — 31.6 
Net amount reclassified to net income (loss)5.0 (0.1)4.9 
Balance, December 31, 2021
(84.0)1.7 (82.3)
Other comprehensive income (loss) before reclassifications(5.9)— (5.9)
Net amount reclassified to net income (loss)(5.7)(0.1)(5.8)
Balance, December 31, 2022
(95.6)1.6 (94.0)
Other comprehensive income (loss) before reclassifications(10.1)— (10.1)
Net amount reclassified to net income (loss)6.3 (0.1)6.2 
Balance, December 31, 2023
$(99.4)$1.5 $(97.9)
Reclassification out of AOCL
Reclassifications out of AOCL were included in the consolidated statements of operations as follows:
Year Ended December 31,
AOCL ComponentsLocation202320222021
Defined benefit plan and other postretirement liabilitiesCost of goods sold$(1.0)$3.5 $26.8 
Other income, net— (8.9)— 
Selling, general and administrative expenses(0.2)3.3 2.9 
Other operating expense (income), net(2.6)(9.0)7.3 
Income tax expense— (0.3)(0.4)
Net of tax$(3.8)$(11.4)$36.6 
Gain (loss) on financial instrumentsCost of goods sold$(0.1)$(0.2)$(0.1)
Income tax effect $— (0.1)0.0 
Net of tax$(0.1)$(0.3)$(0.1)
v3.24.0.1
Pension and Other Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2023
Retirement Benefits [Abstract]  
Schedule of Changes in Projected Benefit Obligations
The change in benefit obligation and change in plan assets as of December 31 are as follows:
PensionOPEB
2023202220232022
Change in benefit obligation:    
Benefit obligation at beginning of year$263.0 $360.1 $73.7 $99.6 
Service cost2.4 4.3 0.1 0.2 
Interest cost14.0 10.3 3.8 2.9 
Plan amendments1.1 — — — 
Actuarial (gain) loss10.1 (87.7)4.5 (23.2)
Medicare Part D— — 0.3 0.2 
Benefits paid(18.8)(24.0)(6.7)(6.1)
Curtailment— — — 0.1 
Benefit obligation at end of year$271.8 $263.0 $75.7 $73.7 
Schedule of Changes in Fair Value of Plan Assets
PensionOPEB
2023202220232022
Change in plan assets:    
Fair value of plan assets at beginning of year$216.6 $329.7 $— $— 
Actual return on plan assets20.7 (93.3)— — 
Employer contributions6.3 4.2 6.4 5.9 
Medicare Part D subsidy received— — 0.3 0.2 
Benefits paid(18.8)(24.0)(6.7)(6.1)
Fair value of assets at end of year$224.8 $216.6 $— $— 
Schedule of Amounts Recognized in Balance Sheet
 PensionOPEB
 2023202220232022
Funded status of plans:    
Funded status$(46.9)$(46.3)$(75.7)$(73.7)
Amounts recognized in the Consolidated Balance Sheets:
Current liabilities(1.8)(1.8)(6.5)(6.1)
Non-current liabilities(45.1)(44.5)(69.2)(67.6)
Net amount recognized$(46.9)$(46.3)$(75.7)$(73.7)
Amounts recognized in accumulated other comprehensive loss (pre-tax):  
Net loss$85.2 $86.7 $14.8 $10.4 
Prior service cost (benefit)1.8 0.8 — — 
Total$87.0 $87.5 $14.8 $10.4 
Schedule of Net Benefit Cost
Net Periodic Benefit Cost:
 Year Ended December 31,
 PensionOPEB
 202320222021202320222021
Service cost$2.4 $4.3 $4.7 $0.1 $0.2 $0.2 
Interest cost14.0 10.3 9.6 3.8 2.9 2.4 
Expected return on plan assets(15.1)(23.5)(22.4)— — — 
Amortization of prior service costs0.1 0.1 0.1 — (1.3)(3.2)
Amortization of net loss6.0 3.5 6.1 0.2 1.3 2.3 
Net periodic benefit cost7.4 (5.3)(1.9)4.1 3.1 1.7 
Curtailment benefit (1)
— — — — (8.9)— 
Total benefit cost$7.4 $(5.3)$(1.9)$4.1 $(5.8)$1.7 
(1)During 2022, we re-measured certain other postretirement benefits triggered by the Hawesville smelter curtailment, leading to a non-cash OPEB curtailment benefit totaling $8.9 million for the year ended December 31, 2022.
Schedule of Amounts Recognized in Other Comprehensive Income (Loss)
Other changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss (pre-tax):
 Year Ended December 31,
 PensionOPEB
 2023202220232022
Net loss (gain)$4.4 $29.0 $4.5 $(23.1)
Prior service cost (benefit)1.2 — — — 
Amortization of net loss, including recognition due to settlement(6.0)(3.5)(0.2)(1.4)
Amortization of prior service (cost) benefit, including curtailment(0.1)(0.1)— 10.3 
Total amount recognized in other comprehensive loss(0.5)25.4 4.3 (14.2)
Net periodic benefit cost7.4 (5.3)4.1 (5.8)
Total recognized in net periodic benefit cost and other comprehensive loss$6.9 $20.1 $8.4 $(20.0)
Schedule of Weighted Average Assumptions Used in Calculating Benefit Obligation and Net Periodic Benefit Cost
Weighted average assumptions used to determine benefit obligations at December 31:
PensionOPEB
2023202220232022
Discount rate (1)
5.19%5.50%5.19%5.57%
Rate of compensation increase (2)
3.5%
4%/3.5%
3.5%
4%/3.5%
Measurement date12/31/202312/31/202212/31/202312/31/2022
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
 PensionOPEB
 202320222021202320222021
Measurement date12/31/202212/31/202112/31/202012/31/202212/31/202112/31/2020
Fiscal year end12/31/202312/31/202212/31/202112/31/202312/31/202212/31/2021
Discount rate (1)
5.50%2.94%2.77%5.57%2.64%1.89%
Rate of compensation increase (2)
4%/3.5%
3%/3.5%
3%/3.5%
4%/3.5%
3%/3.5%
3%/3.5%
Expected return on plan assets (3)
7.25%7.25%7.25%—%—%—%
(1)We use the Ryan Above Median Yield Curve to determine the discount rate.
(2)For 2023, the rate of compensation increase is 4.0% per year for the first year and 3.5% per year thereafter. For 2022 and 2021, the rate of compensation increase is 3.0% per year for the first year and 3.5% per year thereafter.
(3)The rate for each of our defined benefit plans was selected by taking into account our expected asset mix and is based on historical performance as well as expected future rates of return on plan assets.
Schedule of Allocation of Plan Assets The Pension Plans’ weighted average long-term strategic asset allocation policy targets are as follows:
 Pension Plan Asset Allocation
 2023 TargetDecember 31, 2023December 31, 2022
Return seeking assets:
Global equity50%44%46%
Diversified credit15%15%19%
Real assets10%10%14%
Liability hedging assets25%28%21%
Cash—%3%—%
 100%100%100%
Defined Benefit Plan, Plan Assets, Category
The following summarizes the Company’s Pension Plans' assets fair value by asset category:
As of December 31, 2023
Level 1Level 2Level 3Assets measured at NAVTotal
Cash and cash equivalents$— $— $— $4.6 $4.6 
Global Equity— — — 100.0 100.0 
Diversified Credit— — — 34.7 34.7 
Real Assets— — — 22.8 22.8 
Liability hedging assets— — — 62.7 62.7 
Total plan assets fair value$— $— $— $224.8 $224.8 
As of December 31, 2022
Cash and cash equivalents$— $— $— $1.3 $1.3 
Global Equity— — — 99.0 99.0 
Diversified Credit— — — 40.1 40.1 
Real Assets— — — 30.4 30.4 
Liability hedging assets— — — 45.8 45.8 
Total plan assets fair value$— $— $— $216.6 $216.6 
Schedule Of Expected Benefit Plan Contributions
We expect to make the following contributions for 2024:
2024
Expected pension plan contributions$4.5 
Expected OPEB benefits payments6.5 
Schedule of Expected Benefit Payments
The following table provides the estimated future benefit payments for the pension and other postretirement benefit plans:
 Pension BenefitsOPEB Benefits
2024$19.2 $6.5 
202519.4 6.4 
202619.5 6.4 
202719.3 6.4 
202819.5 6.2 
2029 – 203395.9 29.1 
Schedule of Multiemployer Plans
Century’s participation in the plan for the year ended December 31, 2023, is outlined in the table below.
FundSteelworkers Pension Trust
EIN / PN23-6648508 / 499
Pension Protection Act Zone Status 2022 (1)
 Green
Pension Protection Act Zone Status 2021 (1)
 Green
Subject to Financial Improvement/Rehabilitation Plan (2)
 No
Contributions of Century Aluminum 2023$0.2
Contributions of Century Aluminum 2022$1.6
Contributions of Century Aluminum 2021$1.7
Withdrawal from Plan ProbableNo
Surcharge Imposed No
Expiration Date of Collective Bargaining Agreement (2)
March 31, 2026
(1)The most recent Pension Protection Act zone status available in 2023 and 2022 is for the plan's year-end December 31, 2022 and December 31, 2021, respectively. The zone status is based on information that Century received from the plan as well as publicly available information per the Department of Labor and is certified by the plan’s actuary. Among other factors, plans in the green zone are at least 80 percent funded.
(2)The “Subject to Financial Improvement / Rehabilitation Plan” column indicates plans for which a financial improvement plan (FIP) or a rehabilitation plan (RP) is either pending or has been implemented. The last column lists the expiration date(s) of the collective-bargaining agreement(s) to which the plans are subject.
v3.24.0.1
Share-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Summary of service-based share awards activity
Service-based share awardsNumber
Outstanding at January 1, 2023
739,356 
Granted766,929 
Vested(293,404)
Forfeited(56,107)
Outstanding at December 31, 2023
1,156,774 
Performance-based share awardsNumber
Outstanding at January 1, 2023
444,056 
Granted407,808 
Vested(45,605)
Forfeited(203,695)
Outstanding at December 31, 2023
602,564 
 Year ended December 31,
Service-based share awards202320222021
Weighted average per share fair value of service-based share grants$12.58 $17.30 $9.97 
Summary of share-based compensation expense
The following table summarizes the compensation cost recognized for the years ended December 31, 2023, 2022 and 2021 for all service-based and performance-based share awards. The compensation cost is included as part of selling, general and administrative expenses in our Consolidated Statements of Operations.
Year ended December 31,
202320222021
Share-based compensation (benefit) expense reported:   
Performance-based share (benefit) expense$2.0 $(5.0)$12.5 
Service-based share expense4.6 4.4 8.3 
Total share-based compensation (benefit) expense before income tax6.6 (0.6)20.8 
Income tax— — — 
Total share-based compensation (benefit) expense, net of income tax$6.6 $(0.6)$20.8 
v3.24.0.1
Earnings Per Share (Tables)
 12 Months Ended
Dec. 31, 2023
Earnings Per Share [Abstract]  
Schedule of Basic and diluted earnings (loss) per share
The following table shows the basic and diluted earnings (loss) per share for 2023, 2022, and 2021:
For the year ended December 31, 2023
Net Loss
Shares
(in millions)
Per Share
Net loss attributable to Century stockholders$(43.1)
Amount allocated to common stockholders100 %
Basic and Diluted EPS:(1)
$(43.1)92.4 $(0.47)
For the year ended December 31, 2022
Net Loss
Shares
(in millions)
Per Share
Net loss attributable to Century stockholders$(14.1)
Amount allocated to common stockholders100 %
Basic and Diluted EPS:(1)
$(14.1)91.4 $(0.15)
For the year ended December 31, 2021
Net Loss
Shares
(in millions)
Per Share
Net loss attributable to Century stockholders$(167.1)  
Amount allocated to common stockholders100 %  
Basic and Diluted EPS:(1)
$(167.1)90.2 $(1.85)
Schedule of Antidilutive securities excluded from calculation of diluted EPS
Securities excluded from the calculation of diluted EPS (in millions)(1):
2023
2022
2021
Share-based compensation
1.0 1.7 2.7 
Convertible preferred shares5.4 5.8 6.3 
Convertible senior notes4.6 4.6 4.8 
(1)In periods when we report a net loss, all share-based compensation awards, convertible preferred shares and convertible senior notes are excluded from the calculation of diluted weighted average shares outstanding because of their anti-dilutive effect on earnings (loss) per share.
v3.24.0.1
Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2023
Income Tax Disclosure [Abstract]  
Components of pre-tax book income (loss)
The components of pre-tax book income (loss) consist of the following:
Year Ended December 31,
202320222021
U.S.$78.0 $(193.6)$(250.5)
Foreign (144.8)227.0 52.9 
Total $(66.8)$33.4 $(197.6)
Significant components of the income before income tax expense
Significant components of income tax expense consist of the following:
Year Ended December 31,
202320222021
Current:   
U.S. federal current expense (benefit)$0.5 $— $— 
State current expense (benefit)— 0.2 — 
Foreign current expense (benefit)15.8 4.0 0.1 
Total current expense (benefit)16.3 4.2 0.1
Deferred:   
U.S. federal deferred benefit(0.3)(0.3)(0.2)
State deferred benefit(0.1)— — 
Foreign deferred tax (benefit) expense(30.5)43.5 (30.5)
Total deferred (benefit) expense(30.9)43.2 (30.7)
Total income tax (benefit) expense$(14.6)$47.4 $(30.6)
Reconciliation of the statutory U.S. Federal income tax rate to the effective income tax rate on income (loss)
A reconciliation of the statutory U.S. Federal income tax rate to the effective income tax rate on income (loss) is as follows:
Year Ended December 31,
 202320222021
Federal Statutory Rate21.0 %21.0 %21.0 %
Permanent differences1.1 (15.2)(0.3)
State taxes, net of Federal benefit(0.1)0.1 — 
Rate change(0.3)0.4 2.5 
Foreign earnings taxed at different rates than U.S.2.0 (0.8)(3.9)
Valuation allowance3.6 (4.2)(15.9)
Helguvik investment— — 26.4 
Foreign dividends and inclusions(12.7)122.9 (10.1)
Net operating loss expiration and remeasurement(8.0)43.1 (5.2)
Filing differences0.6 (19.1)(0.1)
Changes in uncertain tax reserves(1.3)(5.3)1.3 
Advanced Manufacturing Production Credit18.6 — — 
Other(2.7)(0.9)(0.2)
Effective tax rate21.8 %142.0 %15.5 %
Significant components of deferred tax assets and liabilities
The significant components of our deferred tax assets and liabilities as of December 31 are as follows:
20232022
Deferred tax assets:  
Accrued postretirement benefit cost$30.7 $25.7 
Net operating losses 467.6 395.8 
Disallowed interest expense29.2 27.7 
Derivative and hedging contracts1.2 — 
Fixed asset tax over book basis9.2 17.0 
Other28.3 26.1 
Total deferred tax assets566.2 492.3 
Valuation allowance(537.6)(487.9)
Net deferred tax assets$28.6 $4.4 
Deferred tax liabilities:  
Fixed asset book over tax basis(62.0)(60.5)
Derivatives— (19.0)
Foreign basis differences(18.1)(19.8)
Other(20.6)(7.9)
Total deferred tax liabilities(100.7)(107.2)
Net deferred tax liability$(72.1)$(102.8)
Changes in valuation allowance
The changes in the valuation allowance are as follows:
Year Ended December 31,
202320222021
Beginning balance, valuation allowance$487.9 $485.8 $499.4 
Remeasurement of deferred tax assets— — — 
Release of valuation allowance— — — 
Expiration of net operating losses(7.2)(15.4)(13.2)
Other change in valuation allowance56.9 17.5 (0.4)
Ending balance, valuation allowance$537.6 $487.9 $485.8 
Significant components of net operating loss carryforwards
The significant components of our NOLs are as follows:
20232022
Federal (1)
$1,533.5 $1,487.8 
State (2)
1,221.0 1,182.7 
Foreign (3)(4)
344.4 106.0 
(1)US federal NOLs begin to expire in 2028.
(2)US state NOLs begin to expire in 2027.
(3)NOLs in Iceland expire between 2024 and 2026.
(4)NOLs in Jamaica do not expire.
Reconciliation of beginning and ending amounts of gross unrecognized tax benefits
A reconciliation of the beginning and ending amounts of gross unrecognized tax benefits (excluding interest) is as follows:
202320222021
Balance as of January 1,  $2.2 $4.0 $6.5 
Additions based on tax positions related to the current year1.3 0.3 — 
Decreases due to lapse of applicable statute of limitations(0.5)(2.1)(2.5)
Settlements — — — 
Balance as of December 31,$3.0 $2.2 $4.0 
v3.24.0.1
Commitment and Contingencies (Tables)
 12 Months Ended
Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]  
Schedule of offsetting of financial instruments and derivatives
The following table provides information about the balance sheet location and gross amounts offset:
December 31,
Offsetting of financial instruments and derivativesBalance sheet location20232022
Contingent obligation – principalOther liabilities$(12.9)$(12.9)
Contingent obligation – accrued interestOther liabilities(18.0)(16.6)
Contingent obligation – derivative assetOther liabilities30.9 29.5 
$— $— 
v3.24.0.1
Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2023
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of changes in asset retirement obligations
The reconciliation of the changes in our AROs is presented below:
Year ended December 31,
20232022
Beginning balance$21.2 $20.7 
Additional ARO liabilities incurred3.1 3.8 
ARO liabilities settled(1.5)(4.6)
Accretion expense2.0 1.9 
Acquired ARO liabilities (See Note 2)23.9 — 
Revisions in estimated cash flows2.4 (0.6)
Ending balance$51.1 $21.2 
Current portion of asset retirement obligations (1)
1.6 1.6 
Asset retirement obligations - less current portion$49.5 $19.6 
v3.24.0.1
Business Segments (Tables)
 12 Months Ended
Dec. 31, 2023
Segment Reporting [Abstract]  
Reconciliation of assets from segment to consolidated
A reconciliation of our consolidated assets to the total of primary aluminum segment assets is provided below.
Segment assets (1)
202320222021
Primary$1,808.1 $1,432.4 $1,513.3 
Corporate, unallocated38.4 39.6 56.6 
Total assets$1,846.5 $1,472.0 $1,569.9 
(1)Segment assets include accounts receivable, due from affiliates, prepaid and other current assets, leases - right of use assets, inventory, intangible assets and property, plant and equipment, net; the remaining assets are unallocated corporate assets.
Schedule of revenue from external customers and long-lived assets, by geographical areas Included in the consolidated financial statements are the following amounts related to geographic locations:
 202320222021
Net sales: (1)
   
United States$1,358.6 $1,737.2 $1,413.0 
Iceland826.8 1,040.1 799.5 
Long-lived assets: (2)
   
United States$219.1 $244.9 $400.1 
Iceland529.4 491.0 490.1 
Jamaica275.8 — — 
Other55.1 58.3 61.5 
(1)Includes sales of primary aluminum, scrap aluminum and alumina, and purchased aluminum and alumina.
(2)Includes long-lived assets other than financial instruments and deferred taxes.
Schedule of revenue by major customers by reporting segments A loss of these customers could have a material adverse effect on our results of operations. The net sales related to the customers is as follows:
Year Ended December 31,
202320222021
Glencore$1,612.1 $1,671.1 $1,337.0 
Southwire— 331.3 304.6 
v3.24.0.1
Derivatives (Tables)
 12 Months Ended
Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Schedule of derivative assets and liabilities at fair value
The following table sets forth the Company's derivative assets and liabilities that were accounted for at fair value and not designated as cash flow hedges as of December 31, 2023 and 2022, respectively:
Asset Fair Value
20232022
Commodity contracts (1)
$2.9 $129.1 
Foreign exchange contracts (2)
— — 
Total$2.9 $129.1 
 Liability Fair Value
20232022
Commodity contracts (1)
7.8 23.7 
Foreign exchange contracts (2)
0.1 7.3 
Total$7.9 $31.0 
(1)Commodity contracts reflect our outstanding LME forward financial sales contracts, fixed for floating swaps, Nord Pool power price swaps, HFO price swaps and Indiana Hub power price swaps. At December 31, 2023, $6.4 million of Due to affiliates was related to commodity contract liabilities with Glencore. At December 31, 2022, $11.9 million of Due to affiliates, and $8.3 million of Due to affiliates - less current portion was related to commodity contract liabilities with Glencore.
(2)Foreign exchange contracts reflect our outstanding FX swaps and casthouse currency hedges.
Schedule of net gain (loss) on forward and derivative contracts
The following table summarizes the net gain (loss) on forward and derivative contracts for the years ended December 31, 2023, 2022, and 2021:
Year Ended December 31,
202320222021
Commodity contracts (1)
$63.5 $206.6 $(208.0)
Foreign exchange contracts(1.7)(9.4)(4.4)
   Total$61.8 $197.2 $(212.4)
(1)For the years ended December 31, 2023, 2022, and 2021, $0.6 million, $(13.3) million, and $116.9 million of net gain (loss), respectively, was with Glencore.
v3.24.0.1
Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Property, Plant and Equipment [Line Items]    
Allowance for expected losses $ 0.5 $ 0.5
Minimum | Building and Improvements    
Property, Plant and Equipment [Line Items]    
Useful life 10 years  
Minimum | Machinery and Equipment    
Property, Plant and Equipment [Line Items]    
Useful life 5 years  
Minimum | Technology and Software    
Property, Plant and Equipment [Line Items]    
Useful life 3 years  
Maximum | Building and Improvements    
Property, Plant and Equipment [Line Items]    
Useful life 45 years  
Maximum | Machinery and Equipment    
Property, Plant and Equipment [Line Items]    
Useful life 35 years  
Maximum | Technology and Software    
Property, Plant and Equipment [Line Items]    
Useful life 7 years  
Jamalco    
Schedule of Equity Method Investments [Line Items]    
Ownership percentage 55.00%  
Century Aluminum | Glencore    
Schedule of Equity Method Investments [Line Items]    
Glencore beneficial ownership 42.90%  
Glencore economic ownership 46.00%  
v3.24.0.1
Acquisition of Jamalco - Narrative (Details) - USD ($)
 12 Months Ended
May 02, 2023
Dec. 31, 2023
General Alumina Holdings Limited    
Business Acquisition [Line Items]    
Payments to acquire business $ 1.00  
Jamalco    
Business Acquisition [Line Items]    
Revenue since acquisition   $ 150,300,000
Loss of acquiree since acquisition   41,100,000
Transaction costs   $ 1,400,000
Jamalco    
Business Acquisition [Line Items]    
Ownership percentage   55.00%
Jamalco | General Alumina Holdings Limited    
Business Acquisition [Line Items]    
Ownership percentage 55.00%  
Jamalco | Clarendon Alumina Production Limited    
Business Acquisition [Line Items]    
Glencore beneficial ownership 45.00%  
v3.24.0.1
Acquisition of Jamalco - Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($)
$ in Millions
Dec. 31, 2023
May 02, 2023
Dec. 31, 2022
Business Acquisition [Line Items]      
Deferred credit - preliminary bargain purchase gain $ 273.4   $ 0.0
General Alumina Holdings Limited      
Business Acquisition [Line Items]      
Cash and cash equivalents   $ 19.4  
Restricted cash   8.3  
Accounts receivable - net   7.7  
Non-trade receivables   40.4  
Inventories   103.9  
Prepaid and other current assets   4.2  
Property, plant and equipment   246.2  
Other assets   26.0  
Accounts payable, trade   (94.8)  
Accrued and other current liabilities   (30.8)  
Other liabilities   (35.5)  
Asset retirement obligations   (23.9)  
Total identifiable assets acquired and liabilities assumed   271.1  
Less: noncontrolling interest   (2.3)  
Deferred credit - preliminary bargain purchase gain   $ 273.4  
v3.24.0.1
Acquisition of Jamalco - Unaudited Pro Forma Financial Information (Details) - General Alumina Holdings Limited - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Business Acquisition [Line Items]    
Revenue $ 2,235.1 $ 2,831.0
Earnings $ (45.9) $ (33.1)
v3.24.0.1
Curtailment of Operations - Hawesville (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]      
Asset impairment $ 0.0 $ 159.4 $ 0.0
OPEB curtailment gain, net 0.0 (8.9) $ 0.0
Curtailment Of Operations      
Restructuring Cost and Reserve [Line Items]      
Severance costs 16.6    
Gain (loss) on material sales 1.7    
Curtailment Of Operations | Temporary Facility Closing, Excess Capacity      
Restructuring Cost and Reserve [Line Items]      
Excess capacity charges $ 9.0    
Worker Adjustment and Retraining Notification Act      
Restructuring Cost and Reserve [Line Items]      
Severance costs   18.1  
Hawesville      
Restructuring Cost and Reserve [Line Items]      
Asset impairment   159.4  
OPEB curtailment gain, net   $ 8.9  
v3.24.0.1
Related Party Transactions - Narrative (Details)
carbon_credit in Thousands, € in Millions, $ in Millions
 12 Months Ended
Dec. 18, 2023
EUR (€)
carbon_credit
€ / carbonCredit
Sep. 28, 2023
EUR (€)
carbon_credit
€ / carbonCredit
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 09, 2022
USD ($)
Related Party Transaction [Line Items]            
Net sales | $     $ 2,185.4 $ 2,777.3 $ 2,212.5  
Supply Commitment | Glencore            
Related Party Transaction [Line Items]            
Net sales | $     $ 191.7 $ 24.9 $ 18.3  
Century Aluminum | Glencore            
Related Party Transaction [Line Items]            
Ownership percentage by noncontrolling owners     42.90%      
Economic ownership percentage by related party     46.00%      
Affiliated Entity | Vlissingen Facility Agreement | Vlissingen            
Related Party Transaction [Line Items]            
Credit facility face amount | $           $ 90.0
Stated interest rate           8.75%
Affiliated Entity | Carbon Credit Sale | Nordural Grundartangi ehf            
Related Party Transaction [Line Items]            
Carbon credit sold in transaction | carbon_credit 40 390        
Sale price (Euro per carbon credit) | € / carbonCredit 69.30 82.18        
Aggregate amount | €   € 32.1        
Affiliated Entity | Carbon Credit Repurchase | Nordural Grundartangi ehf            
Related Party Transaction [Line Items]            
Sale price (Euro per carbon credit) | € / carbonCredit 85.13 83.72        
Aggregate amount | € € 33.2 € 32.7        
Affiliated Entity | Additional Carbon Credit Repurchase | Nordural Grundartangi ehf            
Related Party Transaction [Line Items]            
Sale price (Euro per carbon credit) | € / carbonCredit 70.71          
Aggregate amount | € € 2.8          
Customer Concentration Risk | Sales Revenue | Glencore            
Related Party Transaction [Line Items]            
Major customer, percentage revenue, net     73.80% 60.20% 60.00%  
v3.24.0.1
Related Party Transactions (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Related Party Transaction [Line Items]      
Net sales $ 2,185.4 $ 2,777.3 $ 2,212.5
Related Party      
Related Party Transaction [Line Items]      
Net sales 1,612.1 1,671.1 1,337.0
Related Party | Glencore      
Related Party Transaction [Line Items]      
Net sales 1,612.1 1,671.1 1,337.0
Purchases from Glencore $ 181.4 $ 284.7 $ 334.6
v3.24.0.1
Revenue - Disaggregation of Revenue (Details) - USD ($)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disaggregation of Revenue [Line Items]      
Total net sales $ 2,185,400,000 $ 2,777,300,000 $ 2,212,500,000
Contract liability 30,600,000 0  
United States      
Disaggregation of Revenue [Line Items]      
Total net sales 1,358,600,000 1,737,200,000 1,413,000,000
Iceland      
Disaggregation of Revenue [Line Items]      
Total net sales $ 826,800,000 $ 1,040,100,000 $ 799,500,000
v3.24.0.1
Leases (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
May 02, 2023
Lessee, Lease, Description [Line Items]      
Operating lease, weighted average remaining lease term 11 years 9 months 18 days 12 years 2 months 12 days  
Operating lease, weighted average discount rate, percent 7.30% 7.30%  
Right-of-use asset obtained in exchange for operating lease liability $ 3.2 $ 1.7  
Operating lease, payments $ 4.6 $ 4.1  
Jamalco      
Lessee, Lease, Description [Line Items]      
Additional right of use assets from acquisition     $ 2.4
v3.24.0.1
Leases - ROU (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Leases [Abstract]    
ROUA [extensible list] Other assets Other assets
ROUA $ 24.7 $ 20.9
ROUL - current [extensible list] Accrued and other current liabilities Accrued and other current liabilities
ROUL - current $ 2.3 $ 1.8
ROUL -, noncurrent [extensible list] Due to affiliates - less current portion Due to affiliates - less current portion
ROUL - non-current $ 21.9 $ 20.9
ROUL $ 24.2 $ 22.7
v3.24.0.1
Leases - Maturities (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Leases [Abstract]    
2024 $ 3.9  
2025 3.7  
2026 3.3  
2027 2.9  
2028 2.9  
Thereafter 19.4  
Total 36.1  
Less: Interest (11.9)  
ROUL $ 24.2 $ 22.7
v3.24.0.1
Leases - Operating Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Leases [Abstract]    
Operating leases expense $ 4.9 $ 4.5
Short term lease expense 0.6 0.4
Total $ 5.5 $ 4.9
v3.24.0.1
Fair Value Measurements (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
ASSETS:    
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Derivative assets, Other assets  
TOTAL $ 19.9 $ 134.8
LIABILITIES:    
Derivative Liability, Statement of Financial Position [Extensible Enumeration]   Derivative liabilities, Due to affiliates - less current portion
TOTAL 7.9 $ 31.0
Fair Value, Recurring    
ASSETS:    
Cash equivalents 16.8 5.6
Trust assets 0.2 0.1
Derivative instruments 2.9 129.1
LIABILITIES:    
Contingent obligation - net   0.0
Derivative instruments 7.9 31.0
Level 1    
ASSETS:    
TOTAL 17.0 5.7
LIABILITIES:    
TOTAL 0.0 0.0
Level 1 | Fair Value, Recurring    
ASSETS:    
Cash equivalents 16.8 5.6
Trust assets 0.2 0.1
Derivative instruments 0.0 0.0
LIABILITIES:    
Contingent obligation - net   0.0
Derivative instruments 0.0 0.0
Level 2    
ASSETS:    
TOTAL 2.9 127.3
LIABILITIES:    
TOTAL 7.9 26.4
Level 2 | Fair Value, Recurring    
ASSETS:    
Cash equivalents 0.0 0.0
Trust assets 0.0 0.0
Derivative instruments 2.9 127.3
LIABILITIES:    
Contingent obligation - net   0.0
Derivative instruments 7.9 26.4
Level 3    
ASSETS:    
TOTAL 0.0 1.8
LIABILITIES:    
TOTAL 0.0 4.6
Level 3 | Fair Value, Recurring    
ASSETS:    
Cash equivalents 0.0 0.0
Trust assets 0.0 0.0
Derivative instruments 0.0 1.8
LIABILITIES:    
Contingent obligation - net   0.0
Derivative instruments $ 0.0 $ 4.6
v3.24.0.1
Fair Value Measurements - Significant Unobservable Inputs (Details) - Fixed To Variable London Metals Exchange Swap Net
$ in Millions
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Fair value $ 0.0 $ (2.8)
Level 3 | Discount rate net    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, percentage 0.0858 0.0858
v3.24.0.1
Fair Value Measurements - Level 3 Reconciliation (Details) - LME forward financial sales contracts
$ in Millions
 12 Months Ended
Dec. 31, 2023
USD ($)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]  
Balance as of January 1, 2023 $ 1.8
Transfers out of Level 3 (1.8)
Balance as of December 31, 2023 0.0
Change in unrealized gains (losses) 0.0
Balance as of January 1, 2023 (4.6)
Transfers out of Level 3 4.6
Balance as of December 31, 2023 0.0
Change in unrealized gains (losses) $ 0.0
v3.24.0.1
Debt - Activity (Details) - USD ($)
Dec. 31, 2023
Dec. 31, 2022
Apr. 14, 2021
Apr. 09, 2021
Debt Instrument [Line Items]        
Total $ 479,200,000 $ 527,700,000    
Industrial Revenue Bonds, Variable        
Debt Instrument [Line Items]        
Maximum variable interest rate 12.00%      
Industrial revenue bonds due 2028        
Debt Instrument [Line Items]        
Hancock County industrial revenue bonds ("IRBs") due April 2028, interest payable quarterly (variable interest rates (not to exceed 12%)) $ 7,800,000 7,800,000    
Stated interest rate 4.00%      
Senior Notes | Senior Secured Notes, 7.5%        
Debt Instrument [Line Items]        
Financing fees $ 2,600,000      
Stated interest rate 7.50%   7.50%  
7.5% senior secured notes due April 1, 2028, net of financing fees of $2.6 million at December 31, 2023, interest payable semiannually $ 247,400,000 246,600,000    
Senior Notes | Senior Convertible Notes, 2.75%        
Debt Instrument [Line Items]        
Financing fees 1,500,000      
Stated interest rate       2.75%
2.75% convertible senior notes due May 1, 2028, net of financing fees of $1.5 million at December 31, 2023, interest payable semiannually 84,700,000 84,400,000    
U.S revolving credit facility | Revolving Credit Facility        
Debt Instrument [Line Items]        
Revolving credit facility $ 23,700,000 90,000,000.0    
Stated interest rate 9.25%      
Foreign line of credit | Revolving Credit Facility        
Debt Instrument [Line Items]        
Revolving credit facility $ 0 35,000,000.0    
Foreign line of credit | Casthouse Facility        
Debt Instrument [Line Items]        
Revolving credit facility 5,500,000 0    
Financing fees 700,000      
Secured debt $ 98,800,000 49,400,000    
Stated interest rate 8.86%      
Foreign line of credit | Iceland Term Facility        
Debt Instrument [Line Items]        
Revolving credit facility $ 1,300,000 13,300,000    
Financing fees 0      
Secured debt $ 0 1,200,000    
Stated interest rate 3.20%      
Effective interest rate (percent) 7.05%      
Foreign line of credit | Vlissingen Facility Agreement        
Debt Instrument [Line Items]        
Revolving credit facility $ 10,000,000.0 $ 0    
Secured debt $ 10,000,000.0      
Stated interest rate 8.75%      
v3.24.0.1
Debt - Narrative (Details)
$ / shares in Units, € in Millions
 12 Months Ended
Nov. 02, 2021
USD ($)
Apr. 14, 2021
USD ($)
Apr. 09, 2021
USD ($)
$ / shares
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 31, 2023
EUR (€)
Dec. 09, 2022
USD ($)
Sep. 30, 2022
EUR (€)
Sep. 28, 2022
USD ($)
Jun. 14, 2022
USD ($)
Feb. 04, 2022
USD ($)
Nov. 30, 2013
USD ($)
Debt Instrument [Line Items]                          
Proceeds from issuance of Senior Notes due 2028       $ 0 $ 0 $ 250,000,000.0              
Proceeds from issuance of Convertible Senior Notes       $ 0 0 $ 86,300,000              
U.S revolving credit facility                          
Debt Instrument [Line Items]                          
Credit facility face amount                     $ 250,000,000.0    
Letter of Credit                          
Debt Instrument [Line Items]                          
Credit facility face amount                     $ 150,000,000    
Senior Secured Notes, 7.5% | Senior Notes                          
Debt Instrument [Line Items]                          
Stated interest rate   7.50%   7.50%     7.50%            
Face amount   $ 250,000,000                      
Proceeds from issuance of Senior Notes due 2028   $ 245,200,000                      
Redemption price, percentage       100.00%                  
Redemption period       12 months                  
Percentage of outstanding amount, offer to purchase       101.00%     101.00%            
Senior Secured Notes, 7.5% | Senior Notes | Level 2                          
Debt Instrument [Line Items]                          
Fair value of debt instrument       $ 243,500,000                  
Senior Convertible Notes, 2.75% | Senior Notes                          
Debt Instrument [Line Items]                          
Stated interest rate     2.75%                    
Face amount     $ 86,300,000                    
Debt instrument, redemption price, percentage of principal amount redeemed     100.00% 100.00%                  
Proceeds from issuance of Convertible Senior Notes     $ 83,700,000                    
Debt instrument, convertible, conversion ratio     0.0533547                    
Debt Instrument, convertible, conversion price (in dollars per share) | $ / shares     $ 18.74                    
Debt instrument, convertible, threshold percentage of stock price trigger       130.00%                  
Debt instrument, trading days, term       20 days                  
Debt instrument, consecutive trading days, term       30 days                  
Senior Convertible Notes, 2.75% | Senior Notes | Level 2                          
Debt Instrument [Line Items]                          
Fair value of debt instrument       $ 75,300,000                  
Revolving Credit Facility | U.S revolving credit facility                          
Debt Instrument [Line Items]                          
Stated interest rate       9.25%     9.25%            
Revolving credit facility       $ 23,700,000 90,000,000.0                
Outstanding letters of credit issued       61,400,000                  
Revolving Credit Facility | Foreign line of credit                          
Debt Instrument [Line Items]                          
Credit facility face amount                   $ 100,000,000   $ 80,000,000 $ 50,000,000
Revolving credit facility       0 35,000,000.0                
Outstanding letters of credit issued       0                  
Casthouse Facility                          
Debt Instrument [Line Items]                          
Credit facility face amount $ 130,000,000                        
Debt instrument, term 8 years                        
Debt instrument, quarterly installment fee 1.739%                        
Debt instrument, remaining payment after drawdown of funds 60.00%                        
Medium-term notes, outstanding borrowings       $ 104,300,000                  
Debt instrument, arrangement fee, percentage of facility 0.78%                        
Debt instrument, arrangement fee, paid upfront 50.00%                        
Debt instrument, arrangement fee, paid at end of availability 50.00%                        
Line of credit facility, unused capacity, commitment fee percentage 0.38%                        
Casthouse Facility | Foreign line of credit                          
Debt Instrument [Line Items]                          
Stated interest rate       8.86%     8.86%            
Revolving credit facility       $ 5,500,000 0                
Secured debt       98,800,000 49,400,000                
Casthouse Facility | Secured Debt                          
Debt Instrument [Line Items]                          
Long-term debt $ 430,000,000                        
Medium-term Notes                          
Debt Instrument [Line Items]                          
Credit facility face amount | €                 € 13.6        
Medium-term Notes | EURIBOR                          
Debt Instrument [Line Items]                          
Long-term debt       $ 1,300,000     € 1.1            
Iceland Term Facility | Foreign line of credit                          
Debt Instrument [Line Items]                          
Stated interest rate       3.20%     3.20%            
Revolving credit facility       $ 1,300,000 13,300,000                
Secured debt       $ 0 1,200,000                
Vlissingen Facility Agreement | Foreign line of credit                          
Debt Instrument [Line Items]                          
Stated interest rate       8.75%     8.75%            
Revolving credit facility       $ 10,000,000.0 $ 0                
Secured debt       $ 10,000,000.0                  
Vlissingen Facility Agreement | Secured Debt | Vlissingen                          
Debt Instrument [Line Items]                          
Stated interest rate               8.75%          
Face amount               $ 90,000,000          
v3.24.0.1
Debt - Redemption Rights (Details) - Senior secured notes due June 01, 2025 - Long-term Debt
 12 Months Ended
Dec. 31, 2023
2024  
Debt Instrument [Line Items]  
Redemption price, percentage 103.75%
2025  
Debt Instrument [Line Items]  
Redemption price, percentage 101.875%
2026 and Thereafter  
Debt Instrument [Line Items]  
Redemption price, percentage 100.00%
v3.24.0.1
Debt - U.S. Credit Facility Schedule (Details) - U.S revolving credit facility - Revolving Credit Facility
Dec. 31, 2023
USD ($)
Line of Credit Facility [Line Items]  
Credit facility maximum amount $ 250,000,000.0
Borrowing availability 128,800,000
Outstanding letters of credit issued 61,400,000
Outstanding borrowings 23,700,000
Borrowing availability, net of outstanding letters of credit and borrowings $ 43,700,000
v3.24.0.1
Debt - Iceland Credit Facility Schedule (Details) - Foreign line of credit - Revolving Credit Facility
$ in Millions
Dec. 31, 2023
USD ($)
Line of Credit Facility [Line Items]  
Credit facility maximum amount $ 100.0
Borrowing availability 100.0
Outstanding letters of credit issued 0.0
Outstanding borrowings 0.0
Borrowing availability, net of outstanding letters of credit and borrowings $ 100.0
v3.24.0.1
Debt - Industrial Revenue Bonds (Details)
$ in Millions
Dec. 31, 2023
USD ($)
Industrial Revenue Bonds, Variable  
Short-term Debt [Line Items]  
Maximum variable interest rate 12.00%
Surety Bond  
Short-term Debt [Line Items]  
Loss contingency accrual $ 6.6
v3.24.0.1
Shareholders' Equity (Details) - USD ($)
 105 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Mar. 31, 2015
Dec. 31, 2011
Dec. 31, 2008
Class of Stock [Line Items]          
Common stock, shares authorized (shares) 195,000,000 195,000,000      
Common stock, par value (in dollars per share) $ 0.01 $ 0.01      
Common stock, shares issued (shares) 99,876,385 99,510,499      
Common stock, shares outstanding (shares) 92,689,864 92,323,978      
Preferred stock, shares authorized (shares) 5,000,000 5,000,000      
Preferred stock, shares issued (shares)         160,000
Conversion of convertible preferred stock (shares) 100        
Liquidation preference (in dollars per share) $ 0.01        
Authorized repurchase amount $ 130,000,000     $ 60,000,000  
Additional repurchase amount     $ 70,000,000    
Treasury stock, common (in shares) 7,186,521        
Treasury stock, value $ 86,300,000 $ 86,300,000      
Repurchase of common stock (shares) 0        
Remaining authorized repurchase amount $ 43,700,000        
Series A Convertible Preferred Stock          
Class of Stock [Line Items]          
Preferred stock, par value (in dollars per share) $ 0.01 $ 0.01      
Shares, outstanding (in shares) 52,284 53,854      
v3.24.0.1
Inventories (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Inventory, Net [Abstract]    
Raw materials $ 162.5 $ 64.9
Work-in-process 42.9 46.0
Finished goods 46.3 58.0
Operating and other supplies 225.3 229.9
Inventories $ 477.0 $ 398.8
v3.24.0.1
Property, Plant and Equipment (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Property, Plant and Equipment [Abstract]    
Land and improvements $ 105.1 $ 39.8
Mineral Reserves 57.6 0.0
Buildings and improvements 328.8 305.7
Machinery and equipment 1,580.7 1,495.3
Construction in progress 160.3 59.0
Property, plant and equipment, gross 2,232.5 1,899.8
Less accumulated depreciation, amortization and depletion (1,228.3) (1,155.4)
Property, plant and equipment - net $ 1,004.2 $ 744.4
v3.24.0.1
Property, Plant and Equipment (Detail Textual) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Property, Plant and Equipment [Abstract]      
Depreciation, depletion and amortization $ 74.7 $ 73.4 $ 82.6
v3.24.0.1
Accumulated Other Comprehensive Loss (AOCL) - Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect $ (100.2) $ (96.3)
Income tax effect 2.3 2.3
Accumulated other comprehensive loss (97.9) (94.0)
Defined benefit plan and other postretirement liabilities    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect (101.8) (98.0)
Income tax effect 2.6 2.6
Unrealized gain (loss) on financial instruments    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect 1.6 1.7
Income tax effect $ (0.3) $ (0.3)
v3.24.0.1
Accumulated Other Comprehensive Loss (AOCL) - Changes In Components Of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance $ 399.3    
Other comprehensive income (loss) before reclassifications (10.1) $ (5.9) $ 31.6
Net amount reclassified to net income (loss) 6.2 (5.8) 4.9
Ending balance 355.6 399.3  
Cost of goods sold 2,093.5 2,730.6 2,088.3
Other income, net (3.3) 15.3 3.1
Selling, general and administrative expenses 44.3 37.5 57.6
Other operating expense (income), net 15.8 0.0 0.6
Income tax benefit (expense) (14.6) 47.4 (30.6)
Net loss 43.1 14.1 167.1
Defined benefit plan and other postretirement liabilities      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance (95.6) (84.0) (120.6)
Other comprehensive income (loss) before reclassifications (10.1) (5.9) 31.6
Net amount reclassified to net income (loss) 6.3 (5.7) 5.0
Ending balance (99.4) (95.6) (84.0)
Unrealized gain (loss) on financial instruments      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance 1.6 1.7 1.8
Other comprehensive income (loss) before reclassifications 0.0 0.0 0.0
Net amount reclassified to net income (loss) (0.1) (0.1) (0.1)
Ending balance 1.5 1.6 1.7
Accumulated other comprehensive loss      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance (94.0) (82.3) (118.8)
Ending balance (97.9) (94.0) (82.3)
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit plan and other postretirement liabilities      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Cost of goods sold (1.0) 3.5 26.8
Other income, net 0.0 (8.9) 0.0
Selling, general and administrative expenses (0.2) 3.3 2.9
Other operating expense (income), net (2.6) (9.0) 7.3
Income tax benefit (expense) 0.0 (0.3) (0.4)
Net loss (3.8) (11.4) 36.6
Reclassification out of Accumulated Other Comprehensive Income | Unrealized gain (loss) on financial instruments      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Cost of goods sold (0.1) (0.2) (0.1)
Income tax benefit (expense) 0.0 (0.1) 0.0
Net loss $ (0.1) $ (0.3) $ (0.1)
v3.24.0.1
Pension and Other Postretirement Benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year $ 263.0 $ 360.1  
Service cost 2.4 4.3 $ 4.7
Interest cost 14.0 10.3 9.6
Plan amendments 1.1 0.0  
Actuarial (gain) loss 10.1 (87.7)  
Medicare Part D 0.0 0.0  
Benefits paid (18.8) (24.0)  
Curtailment 0.0 0.0  
Benefit obligation at end of year 271.8 263.0 360.1
OPEB      
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 73.7 99.6  
Service cost 0.1 0.2 0.2
Interest cost 3.8 2.9 2.4
Plan amendments 0.0 0.0  
Actuarial (gain) loss 4.5 (23.2)  
Medicare Part D 0.3 0.2  
Benefits paid (6.7) (6.1)  
Curtailment 0.0 0.1  
Benefit obligation at end of year $ 75.7 $ 73.7 $ 99.6
v3.24.0.1
Pension and Other Postretirement Benefits - Change in Plan Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension    
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year $ 216.6 $ 329.7
Actual return on plan assets 20.7 (93.3)
Employer contributions 6.3 4.2
Medicare Part D subsidy received 0.0 0.0
Benefits paid (18.8) (24.0)
Fair value of assets at end of year 224.8 216.6
OPEB    
Change in fair value of plan assets [Roll Forward]    
Fair value of plan assets at beginning of year 0.0 0.0
Actual return on plan assets 0.0 0.0
Employer contributions 6.4 5.9
Medicare Part D subsidy received 0.3 0.2
Benefits paid (6.7) (6.1)
Fair value of assets at end of year $ 0.0 $ 0.0
v3.24.0.1
Pension and Other Postretirement Benefits - Funded status of plans (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Pension    
Defined Benefit Plan Disclosure [Line Items]    
Funded status $ (46.9) $ (46.3)
Current liabilities (1.8) (1.8)
Non-current liabilities (45.1) (44.5)
Net amount recognized (46.9) (46.3)
Net loss 85.2 86.7
Prior service cost (benefit) 1.8 0.8
Total 87.0 87.5
Projected benefit obligation for plans with accumulated benefit obligations in excess of plan assets 271.8 263.0
Accumulated benefit obligation for plans with accumulated benefit obligations in excess of plan assets 267.6 258.8
Fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets 224.8 216.6
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Funded status (75.7) (73.7)
Current liabilities (6.5) (6.1)
Non-current liabilities (69.2) (67.6)
Net amount recognized (75.7) (73.7)
Net loss 14.8 10.4
Prior service cost (benefit) 0.0 0.0
Total $ 14.8 $ 10.4
v3.24.0.1
Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 2.4 $ 4.3 $ 4.7
Interest cost 14.0 10.3 9.6
Expected return on plan assets (15.1) (23.5) (22.4)
Amortization of prior service costs 0.1 0.1 0.1
Amortization of net loss 6.0 3.5 6.1
Net periodic benefit cost 7.4 (5.3) (1.9)
Curtailment benefit 0.0 0.0 0.0
Total benefit cost 7.4 (5.3) (1.9)
OPEB      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.1 0.2 0.2
Interest cost 3.8 2.9 2.4
Expected return on plan assets 0.0 0.0 0.0
Amortization of prior service costs 0.0 (1.3) (3.2)
Amortization of net loss 0.2 1.3 2.3
Net periodic benefit cost 4.1 3.1 1.7
Curtailment benefit 0.0 (8.9) 0.0
Total benefit cost $ 4.1 $ (5.8) $ 1.7
v3.24.0.1
Pension and Other Postretirement Benefits - Changes in Plan Assets & Benefit Obligation Recognized in OCI (pre-taxed) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]      
Net loss (gain) $ 10.1 $ 5.9 $ (31.6)
Amortization of prior service (cost) benefit, including curtailment (0.1) 1.2 3.1
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Net loss (gain) 4.4 29.0  
Prior service cost (benefit) 1.2 0.0  
Amortization of net loss, including recognition due to settlement (6.0) (3.5)  
Amortization of prior service (cost) benefit, including curtailment (0.1) (0.1)  
Total amount recognized in other comprehensive loss (0.5) 25.4  
Total benefit cost 7.4 (5.3) (1.9)
Total recognized in net periodic benefit cost and other comprehensive loss 6.9 20.1  
OPEB      
Defined Benefit Plan Disclosure [Line Items]      
Net loss (gain) 4.5 (23.1)  
Prior service cost (benefit) 0.0 0.0  
Amortization of net loss, including recognition due to settlement (0.2) (1.4)  
Amortization of prior service (cost) benefit, including curtailment 0.0 10.3  
Total amount recognized in other comprehensive loss 4.3 (14.2)  
Total benefit cost 4.1 (5.8) $ 1.7
Total recognized in net periodic benefit cost and other comprehensive loss $ 8.4 $ (20.0)  
v3.24.0.1
Pension and Other Postretirement Benefits - Weighted Average assumptions used in calculating benefit obligations (Details)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 5.19% 5.50%
Rate of compensation increase, group 1 3.50% 4.00%
Rate of compensation increase, group 2   3.50%
Measurement date Dec. 31, 2023 Dec. 31, 2022
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 5.19% 5.57%
Rate of compensation increase, group 1 3.50% 4.00%
Rate of compensation increase, group 2   3.50%
Measurement date Dec. 31, 2023 Dec. 31, 2022
v3.24.0.1
Pension and Other Postretirement Benefits - Weighted average assumptions used to determine net periodic benefit cost (Details)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Rate of compensation increase, year one 4.00% 3.00% 3.00%
Rate of compensation increase, year one and thereafter 3.50% 3.50% 3.50%
Pension      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2022 12/31/2021 12/31/2020
Fiscal year end 12/31/2023 12/31/2022 12/31/2021
Discount rate 5.50% 2.94% 2.77%
Rate of compensation increase, group 1 4.00% 3.00% 3.00%
Rate of compensation increase, group 2 3.50% 3.50% 3.50%
Expected return on plan assets 7.25% 7.25% 7.25%
OPEB      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2022 12/31/2021 12/31/2020
Fiscal year end 12/31/2023 12/31/2022 12/31/2021
Discount rate 5.57% 2.64% 1.89%
Rate of compensation increase, group 1 4.00% 3.00% 3.00%
Rate of compensation increase, group 2 3.50% 3.50% 3.50%
Expected return on plan assets 0.00% 0.00% 0.00%
v3.24.0.1
Pension and Other Postretirement Benefits - Assumed health care trend rates (Details)
Dec. 31, 2023
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]  
Medical cost inflation, years 13 and thereafter 4.50%
Pre 65  
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]  
Medical cost inflation rate 7.00%
Post 65  
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]  
Medical cost inflation rate 6.50%
v3.24.0.1
Pension and Other Postretirement Benefits - Pension Plan Asset Allocation (Details)
Dec. 31, 2023
Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]    
Target allocation percentage of assets 100.00%  
Actual plan asset allocations 100.00% 100.00%
Global equity    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation percentage of assets 50.00%  
Actual plan asset allocations 44.00% 46.00%
Diversified credit    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation percentage of assets 15.00%  
Actual plan asset allocations 15.00% 19.00%
Real assets    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation percentage of assets 10.00%  
Actual plan asset allocations 10.00% 14.00%
Liability hedging assets    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation percentage of assets 25.00%  
Actual plan asset allocations 28.00% 21.00%
Cash    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation percentage of assets 0.00%  
Actual plan asset allocations 3.00% 0.00%
v3.24.0.1
Pension and Other Postretirement Benefits - Fair Value Hierarchy (Details) - Pension - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets $ 224.8 $ 216.6 $ 329.7
Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Assets measured at NAV      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 224.8 216.6  
Cash      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 4.6 1.3  
Cash | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Cash | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Cash | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Cash | Assets measured at NAV      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 4.6 1.3  
Global equity      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 100.0 99.0  
Global equity | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Global equity | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Global equity | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Global equity | Assets measured at NAV      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 100.0 99.0  
Diversified credit      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 34.7 40.1  
Diversified credit | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Diversified credit | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Diversified credit | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Diversified credit | Assets measured at NAV      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 34.7 40.1  
Real assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 22.8 30.4  
Real assets | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Real assets | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0    
Real assets | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Real assets | Assets measured at NAV      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 22.8 30.4  
Liability hedging assets      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 62.7 45.8  
Liability hedging assets | Level 1      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Liability hedging assets | Level 2      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Liability hedging assets | Level 3      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets 0.0 0.0  
Liability hedging assets | Assets measured at NAV      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of assets $ 62.7 $ 45.8  
v3.24.0.1
Pension and Other Postretirement Benefits - Expected Contribution Cash Flows (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension    
Defined Benefit Plan Disclosure [Line Items]    
Employer contributions $ 6.3 $ 4.2
Expected contributions in next twelve months 4.5  
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]    
2024 19.2  
2025 19.4  
2026 19.5  
2027 19.3  
2028 19.5  
2029 – 2033 95.9  
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Employer contributions 6.4 $ 5.9
Expected contributions in next twelve months 6.5  
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]    
2024 6.5  
2025 6.4  
2026 6.4  
2027 6.4  
2028 6.2  
2029 – 2033 $ 29.1  
v3.24.0.1
Pension and Other Postretirement Benefits - Participation in Multi-Employer Pension Plans (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Retirement Benefits [Abstract]      
EIN 236648508    
PN 499    
Pension protection act zone status   Green Green
Subject to financial improvement/rehabilitation plan No    
Contributions of century aluminum company $ 0.2 $ 1.6 $ 1.7
Withdrawal from Plan Probable No    
Surcharge Imposed No    
Expiration date of collective bargaining agreement Mar. 31, 2026    
v3.24.0.1
Pension and Other Postretirement Benefits - Company matching contribution to defined contribution 401(K) plans (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Retirement Benefits [Abstract]      
Age requirement for future accruals 50 years    
Company matching contribution to defined contribution (401(k)) plans $ 5.8 $ 6.0 $ 5.3
v3.24.0.1
Share-based Compensation (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2023
USD ($)
award_type
shares
Dec. 31, 2022
USD ($)
Amended and Restated Stock Option Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share based compensation, vesting rights (in time period) 3 years  
Shares authorized (in shares) 12,900,000  
Shares remaining (in shares) 3,659,885  
Amended and Restated Stock Option Plan | Non-employee director    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share based compensation, vesting rights (in time period) 12 months  
Long Term Incentive Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share based compensation, vesting rights (in time period) 3 years  
Number of award types provided | award_type 2  
Performance unit liability | $ $ 3.3 $ 1.7
v3.24.0.1
Share-based Compensation - Service Based Awards Rollforward (Details)
 12 Months Ended
Dec. 31, 2023
shares
Service-Based Share Awards  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
Service-based share awards outstanding, beginning of period (in shares) 739,356
Granted (in shares) 766,929
Vested (in shares) (293,404)
Forfeited (in shares) (56,107)
Service-based share awards outstanding, end of period (in shares) 1,156,774
Performance-based share awards  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]  
Service-based share awards outstanding, beginning of period (in shares) 444,056
Granted (in shares) 407,808
Vested (in shares) (45,605)
Forfeited (in shares) (203,695)
Service-based share awards outstanding, end of period (in shares) 602,564
v3.24.0.1
Share-based Compensation - Service-Based Awards, Additional disclosures (Details) - $ / shares
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Service-Based Awards, Additional disclosures [Abstract]      
Weighted average per share fair value of service-based share grants (in dollars per share) $ 12.58 $ 17.30 $ 9.97
v3.24.0.1
Share-based Compensation - Share and performance-based compensation expense (Details) - USD ($)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Share and performance-based compensation expense [Abstract]      
Performance-based share (benefit) expense $ 2,000,000.0 $ (5,000,000.0) $ 12,500,000
Service-based share expense 4,600,000 4,400,000 8,300,000
Total share-based compensation (benefit) expense before income tax 6,600,000 (600,000) 20,800,000
Income tax 0 0 0
Total share-based compensation (benefit) expense, net of income tax 6,600,000 (600,000) 20,800,000
Share-based payment arrangement, amount capitalized 0 $ 0 $ 0
Unrecognized compensation expense $ 10,400,000    
Weighted average period of expense recognition 1 year 9 months 18 days    
v3.24.0.1
Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Earnings Per Share [Abstract]      
Net loss attributable to Century stockholders $ (43.1) $ (14.1) $ (167.1)
Amount allocated to common stockholders 100.00% 100.00% 100.00%
Basic and Diluted EPS:      
Net income (loss) allocated to common stockholders $ (43.1) $ (14.1) $ (167.1)
Net income (loss) available to common stockholders, diluted $ (43.1) $ (14.1) $ (167.1)
Basic (in shares) 92,400 91,400 90,200
Diluted (in shares) 92,400 91,400 90,200
Basic (in dollars per share) $ (0.47) $ (0.15) $ (1.85)
Diluted (in dollars per share) $ (0.47) $ (0.15) $ (1.85)
v3.24.0.1
Earnings Per Share - Antidilutive securities excluded from the calculation of diluted EPS (Details) - shares
shares in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Share-based compensation      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Share based compensation (in shares) 1.0 1.7 2.7
Convertible preferred shares      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Share based compensation (in shares) 5.4 5.8 6.3
Convertible senior notes      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Share based compensation (in shares) 4.6 4.6 4.8
v3.24.0.1
Income Taxes - Components of Pre-tax Book Income (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Income Tax Disclosure [Abstract]      
U.S. $ 78.0 $ (193.6) $ (250.5)
Foreign  (144.8) 227.0 52.9
(Loss) income before income taxes $ (66.8) $ 33.4 $ (197.6)
v3.24.0.1
Income Taxes - Significant Components of Income Tax Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Current:      
U.S. federal current expense (benefit) $ 0.5 $ 0.0 $ 0.0
State current expense (benefit) 0.0 0.2 0.0
Foreign current expense (benefit) 15.8 4.0 0.1
Total current expense (benefit) 16.3 4.2 0.1
Deferred:      
U.S. federal deferred benefit (0.3) (0.3) (0.2)
State deferred benefit (0.1) 0.0 0.0
Foreign deferred tax (benefit) expense (30.5) 43.5 (30.5)
Total deferred (benefit) expense (30.9) 43.2 (30.7)
Total income tax (benefit) expense $ (14.6) $ 47.4 $ (30.6)
v3.24.0.1
Income Taxes - Reconciliation of statutory to effective income tax rate (Details)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Effective Income Tax Rate Reconciliation, Amount [Abstract]      
Federal Statutory Rate 21.00% 21.00% 21.00%
Permanent differences 1.10% (15.20%) (0.30%)
State taxes, net of Federal benefit (0.10%) 0.10% 0.00%
Rate change (0.30%) 0.40% 2.50%
Foreign earnings taxed at different rates than U.S. 2.00% (0.80%) (3.90%)
Valuation allowance 3.60% (4.20%) (15.90%)
Helguvik investment 0.00% 0.00% 26.40%
Foreign dividends and inclusions (12.70%) 122.90% (10.10%)
Net operating loss expiration and remeasurement (8.00%) 43.10% (5.20%)
Filing differences 0.60% (19.10%) (0.10%)
Changes in uncertain tax reserves (1.30%) (5.30%) 1.30%
Advanced Manufacturing Production Credit 18.60% 0.00% 0.00%
Other (2.70%) (0.90%) (0.20%)
Effective tax rate 21.80% 142.00% 15.50%
v3.24.0.1
Income Taxes - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Tax Credit Carryforward [Line Items]        
Reduction in cost of goods sold $ (2,093.5) $ (2,730.6) $ (2,088.3)  
Reduction in selling, general and administrative expenses (44.3) (37.5) (57.6)  
Valuation allowance 537.6 487.9    
Unrecognized tax benefits 3.0 $ 2.2 $ 4.0 $ 6.5
Unrecognized tax benefits that would impact effective tax rate 1.4      
Inflation Reduction Act        
Tax Credit Carryforward [Line Items]        
Reduction in cost of goods sold 56.5      
Reduction in selling, general and administrative expenses $ 2.8      
v3.24.0.1
Income Taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Deferred tax assets:    
Accrued postretirement benefit cost $ 30.7 $ 25.7
Net operating losses 467.6 395.8
Disallowed interest expense 29.2 27.7
Derivative and hedging contracts 1.2 0.0
Fixed asset tax over book basis 9.2 17.0
Other 28.3 26.1
Total deferred tax assets 566.2 492.3
Valuation allowance (537.6) (487.9)
Net deferred tax assets 28.6 4.4
Deferred tax liabilities:    
Fixed asset book over tax basis (62.0) (60.5)
Derivatives 0.0 (19.0)
Foreign basis differences (18.1) (19.8)
Other (20.6) (7.9)
Total deferred tax liabilities (100.7) (107.2)
Net deferred tax liability $ (72.1) $ (102.8)
v3.24.0.1
Income Taxes - Changes in Valuation Allowance (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance, valuation allowance $ 487.9 $ 485.8 $ 499.4
Remeasurement of deferred tax assets 0.0 0.0 0.0
Release of valuation allowance 0.0 0.0 0.0
Expiration of net operating losses (7.2) (15.4) (13.2)
Other change in valuation allowance 56.9 17.5 (0.4)
Balance, valuation allowance $ 537.6 $ 487.9 $ 485.8
v3.24.0.1
Income Taxes - Net Operating Losses Carryforwards (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Federal    
Operating Loss Carryforwards [Line Items]    
Operating loss carryforwards $ 1,533.5 $ 1,487.8
State    
Operating Loss Carryforwards [Line Items]    
Operating loss carryforwards 1,221.0 1,182.7
Foreign    
Operating Loss Carryforwards [Line Items]    
Operating loss carryforwards $ 344.4 $ 106.0
v3.24.0.1
Income Taxes - Gross Unrecognized Tax Positions (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance as of January 1,   $ 2.2 $ 4.0 $ 6.5
Additions based on tax positions related to the current year 1.3 0.3 0.0
Decreases due to lapse of applicable statute of limitations (0.5) (2.1) (2.5)
Settlements  0.0 0.0 0.0
Balance as of December 31, $ 3.0 $ 2.2 $ 4.0
v3.24.0.1
Commitments and Contingencies - Legal Contingencies (Details)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 27 Months Ended
Jan. 01, 2024
Oct. 01, 2021
USD ($)
Aug. 18, 2017
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Dec. 31, 2023
USD ($)
MW
Dec. 31, 2013
USD ($)
Dec. 31, 2023
USD ($)
MW
Sep. 30, 2022
MW
Jul. 31, 2021
MW
Ravenswood Retiree Medical Benefits Changes                          
Loss Contingencies [Line Items]                          
Litigation settlement amount     $ 23,000,000                    
Litigation settlement period                 10 years        
Ravenswood retiree legal settlement       $ 5,000,000                  
Gain (loss) related to litigation settlement       5,500,000                  
Loss contingency accrual       $ 12,500,000                  
Litigation settlement, annual installment                 $ 2,000,000        
Litigation settlement installment period                 9 years        
Loss contingency, accrual, current                 $ 2,000,000   $ 2,000,000    
Other liabilities                 $ 3,300,000   3,300,000    
Pension Benefit Guarantee Corporation                          
Loss Contingencies [Line Items]                          
Required pension contributions above minimum                   $ 17,400,000      
Pension contributions         $ 0 $ 0 $ 0 $ 0     $ 6,900,000    
Pension contributions, amended term, annual contribution   $ 2,400,000                      
Pension contributions, amended term, total contribution   $ 9,600,000                      
Pension contributions, term   4 years                      
Grundartangi - HS, Landsvirkjun                          
Loss Contingencies [Line Items]                          
Underlying, derivative power, available | MW                 545   545    
Grundartangi - Landsvirkjun                          
Loss Contingencies [Line Items]                          
Underlying, derivative power, available | MW                       25  
Underlying, derivative power, available, extension | MW                       42 161
Underlying, derivative power, available, requested | MW                         182
Underlying, derivative power, rate | MW                       119  
Subsequent Event | Mt. Holly                          
Loss Contingencies [Line Items]                          
Short-term power agreement, supply percentage 100.00%                        
v3.24.0.1
Commitments and Contingencies - Other Commitments (Details)
Dec. 31, 2023
laborUnion
Loss Contingencies [Line Items]  
Percentage of work force represented by a union 60.00%
Percentage of U.S. based work force represented by a union 42.00%
Percentage of foreign work force represented by union 0.62
Netherlands  
Loss Contingencies [Line Items]  
Percentage of work force represented by a union 100.00%
Grundartangi - Landsvirkjun  
Loss Contingencies [Line Items]  
Percentage of foreign work force covered under new labor agreement 87.00%
Number of labor unions under new agreement 5
v3.24.0.1
Commitments and Contingencies - Contingent obligation (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2023
USD ($)
paymentInstallment
Dec. 31, 2022
USD ($)
Debt Instrument [Line Items]    
Contingent obligation $ 0.0 $ 0.0
E.ON Contingent Obligation | Other Liabilities    
Debt Instrument [Line Items]    
Contingent obligation – principal (12.9) (12.9)
Contingent obligation – accrued interest (18.0) (16.6)
Contingent obligation – derivative asset $ 30.9 $ 29.5
Long-term Debt | E.ON Contingent Obligation    
Debt Instrument [Line Items]    
Number of monthly installments | paymentInstallment 72  
Stated interest rate 10.94%  
Realized gain (loss) on contingent obligation $ 1.4  
v3.24.0.1
Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Asset Retirement Obligation [Roll Forward]    
Beginning balance $ 21.2 $ 20.7
Additional ARO liabilities incurred 3.1 3.8
ARO liabilities settled (1.5) (4.6)
Accretion expense 2.0 1.9
Acquired ARO liabilities 23.9 0.0
Revisions in estimated cash flows 2.4 (0.6)
Ending balance 51.1 21.2
Current portion of asset retirement obligations 1.6 1.6
Asset retirement obligations - less current portion $ 49.5 $ 19.6
v3.24.0.1
Business Segments (Details)
 12 Months Ended
Dec. 31, 2023
segment
Segment, Reconciliation of Other Items from Segments to Consolidated [Line Items]  
Number of operating segments 5
Number of reportable segments 1
Bauxite Mining and Alumina Refinery Joint Venture  
Segment, Reconciliation of Other Items from Segments to Consolidated [Line Items]  
Ownership percentage 55.00%
v3.24.0.1
Business Segments - Segment Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]      
Assets $ 1,846.5 $ 1,472.0 $ 1,569.9
Primary      
Segment Reporting, Asset Reconciling Item [Line Items]      
Assets 1,808.1 1,432.4 1,513.3
Corporate, unallocated      
Segment Reporting, Asset Reconciling Item [Line Items]      
Assets $ 38.4 $ 39.6 $ 56.6
v3.24.0.1
Business Segments - Geography, Segment Reporting (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]      
Total net sales $ 2,185.4 $ 2,777.3 $ 2,212.5
United States      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Total net sales 1,358.6 1,737.2 1,413.0
Long-lived assets 219.1 244.9 400.1
Iceland      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Total net sales 826.8 1,040.1 799.5
Long-lived assets 529.4 491.0 490.1
Jamaica      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Long-lived assets 275.8 0.0 0.0
Other      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Long-lived assets $ 55.1 $ 58.3 $ 61.5
v3.24.0.1
Business Segments - Major Customers (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Glencore      
Revenue, Major Customer [Line Items]      
Revenues $ 1,612.1 $ 1,671.1 $ 1,337.0
Southwire      
Revenue, Major Customer [Line Items]      
Revenues $ 0.0 $ 331.3 $ 304.6
v3.24.0.1
Derivatives - Narrative (Details)
€ in Millions, kr in Millions
 12 Months Ended
Dec. 31, 2023
DKK (kr)
MMBTU
MWh
Boe
t
Dec. 31, 2023
EUR (€)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Supply Commitment [Line Items]        
Restricted cash | $     $ 0 $ 0
LME forward financial sales contracts        
Supply Commitment [Line Items]        
Open position to offset fixed prices | t 36,633      
Fixed to Floating Swap        
Supply Commitment [Line Items]        
Open position to offset fixed prices | t 318      
Nord Pool Swaps | Nordural Grundartangi ehf        
Supply Commitment [Line Items]        
Derivative liability (in MwH) | MWh 0      
Indiana Hub Power Price Swaps        
Supply Commitment [Line Items]        
Derivative liability (in MwH) | MWh 0      
USD ISK Forward Swap | Not Designated as Hedging Instrument | Short        
Supply Commitment [Line Items]        
Derivative, forward contracts | kr kr 397.0      
USD Euro Forward Swap | Not Designated as Hedging Instrument | Short        
Supply Commitment [Line Items]        
Derivative, forward contracts | €   € 1.8    
Henry Hub Natural Gas Price Swaps        
Supply Commitment [Line Items]        
Derivative liability (in MwH) | MMBTU 0      
HFO Price Swaps        
Supply Commitment [Line Items]        
Derivative liability (in MwH) | Boe 180,000      
v3.24.0.1
Derivative - Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Derivative [Line Items]    
Due to affiliates - less current portion $ 66.3 $ 31.4
Related Party    
Derivative [Line Items]    
Due to affiliates 101.4 17.0
Commodity Contract | Related Party    
Derivative [Line Items]    
Due to affiliates 6.4 11.9
Due to affiliates - less current portion   8.3
Not Designated as Hedging Instrument    
Derivative [Line Items]    
Asset Fair Value 2.9 129.1
Liability Fair Value 7.9 31.0
Not Designated as Hedging Instrument | Commodity Contract    
Derivative [Line Items]    
Asset Fair Value 2.9 129.1
Liability Fair Value 7.8 23.7
Not Designated as Hedging Instrument | Foreign Exchange Contract    
Derivative [Line Items]    
Asset Fair Value 0.0 0.0
Liability Fair Value $ 0.1 $ 7.3
v3.24.0.1
Derivative - Net Gain (Loss) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Derivative [Line Items]      
Derivative, gain (loss) on derivative, net $ 61.8 $ 197.2 $ (212.4)
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Net (loss) gain on forward and derivative contracts    
Commodity Contract      
Derivative [Line Items]      
Derivative, gain (loss) on derivative, net $ 63.5 206.6 (208.0)
Commodity Contract | Glencore      
Derivative [Line Items]      
Derivative, gain (loss) on derivative, net 0.6 (13.3) 116.9
Foreign Exchange Contract      
Derivative [Line Items]      
Derivative, gain (loss) on derivative, net $ (1.7) $ (9.4) $ (4.4)