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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.