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Pension and Other Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2022
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
 2022202120222021
Change in benefit obligation:    
Benefit obligation at beginning of year$360.1 $385.8 $99.6 $107.9 
Service cost4.3 4.7 0.2 0.2 
Interest cost10.3 9.6 2.9 2.4 
Actuarial (gain) loss(87.7)(18.8)(23.2)(5.0)
Medicare Part D— — 0.2 0.2 
Benefits paid(24.0)(21.2)(6.1)(6.1)
Curtailment— — 0.1 — 
Benefit obligation at end of year$263.0 $360.1 $73.7 $99.6 
Schedule of Changes in Fair Value of Plan Assets
 PensionOPEB
 2022202120222021
Change in plan assets:    
Fair value of plan assets at beginning of year$329.7 $318.8 $— $— 
Actual return on plan assets(93.3)30.3 — — 
Employer contributions4.2 1.8 5.9 5.9 
Medicare Part D subsidy received— — 0.2 0.2 
Benefits paid(24.0)(21.2)(6.1)(6.1)
Fair value of assets at end of year$216.6 $329.7 $— $— 
The decrease of actual return on plan assets in 2022 was primarily attributable to fluctuations in market prices during the year.
Schedule of Amounts Recognized in Balance Sheet
 PensionOPEB
 2022202120222021
Funded status of plans:    
Funded status$(46.3)$(30.4)$(73.7)$(99.6)
Amounts recognized in the Consolidated Balance Sheets:
Current liabilities(1.8)(1.8)(6.1)(6.3)
Non-current liabilities(44.5)(28.6)(67.6)(93.3)
Net amount recognized$(46.3)$(30.4)$(73.7)$(99.6)
Amounts recognized in accumulated other comprehensive loss (pre-tax):  
Net loss$86.7 $61.2 $10.4 $35.0 
Prior service cost (benefit)0.8 0.9 0.0 (10.3)
Total$87.5 $62.1 $10.4 $24.7 
Schedule of Net Benefit Cost
Net Periodic Benefit Cost:
 Year Ended December 31,
 PensionOPEB
 202220212020202220212020
Service cost$4.3 $4.7 $4.9 $0.2 $0.2 $0.2 
Interest cost10.3 9.6 11.4 2.9 2.4 3.1 
Expected return on plan assets(23.5)(22.4)(20.8)— — — 
Amortization of prior service costs0.1 0.1 0.1 (1.3)(3.2)(3.2)
Amortization of net loss3.5 6.1 6.5 1.3 2.3 2.2 
Net periodic benefit cost(5.3)(1.9)2.1 3.1 1.7 2.3 
Curtailment benefit (1)
— — — (8.9)— — 
Total benefit cost$(5.3)$(1.9)$2.1 $(5.8)$1.7 $2.3 
(1) During the third quarter, 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
 2022202120222021
Net loss (gain)$29.0 $(26.7)$(23.1)$(5.0)
Amortization of net loss, including recognition due to settlement(3.5)(6.1)(1.4)(2.3)
Amortization of prior service (cost) benefit, including curtailment(0.1)(0.1)10.3 3.2 
Total amount recognized in other comprehensive loss25.4 (32.9)(14.2)(4.1)
Net periodic benefit cost(5.3)(1.9)(5.8)1.7 
Total recognized in net periodic benefit cost and other comprehensive loss$20.1 $(34.8)$(20.0)$(2.4)
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
 2022202120222021
Discount rate (1)
5.50%2.89%5.57%2.75%
Rate of compensation increase (2)
4%/3.5%
3%/3.5%
4%/3.5%
3%/3.5%
Measurement date12/31/202212/31/202112/31/202212/31/2021


Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
 PensionOPEB
 202220212020202220212020
Measurement date12/31/202112/31/202012/31/201912/31/202112/31/202012/31/2019
Fiscal year end12/31/202212/31/202112/31/202012/31/202212/31/202112/31/2020
Discount rate (1)
2.94%2.77%3.22 %2.64 %1.89 %2.79 %
Rate of compensation increase (2)
3%/3.5%
3%/3.5%
3%/3.5%
3%/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 2022, 2021, and 2020, the rate of compensation increase is 3% 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
 2022 TargetDecember 31, 2022December 31, 2021
Return seeking assets:
Global equity50%46%51%
Diversified credit15%19%15%
Real assets10%14%9%
Liability hedging assets25%21%25%
Cash—%—%—%
 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, 2022Level 1Level 2Level 3Assets measured at NAVTotal
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 $216.6 
As of December 31, 2021
Cash and cash equivalents$— $— $— $2.5 $2.5 
Global Equity— — — 167.8 167.8 
Diversified Credit— — — 49.2 49.2 
Real Assets— — — 28.6 28.6 
Liability hedging assets— — — 81.6 81.6 
Total plan assets fair value$— $— $— $329.7 $329.7 
Schedule Of Expected Benefit Plan Contributions We expect to make the following contributions for 2023:
 2023
Expected pension plan contributions$4.2 
Expected OPEB benefits payments6.1 
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
2023$20.2 $6.1 
202419.9 6.2 
202519.9 6.1 
202620.0 6.0 
202719.7 6.0 
2028 – 203297.4 28.8 
Schedule of Multiemployer Plans
Century’s participation in the plan for the year ended December 31, 2022, 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 2022$1.6
Contributions of Century Aluminum 2021$1.7
Contributions of Century Aluminum 2020$1.4
Withdrawal from Plan ProbableNo
Surcharge ImposedNo
Expiration Date of Collective Bargaining Agreement (2)
March 31, 2026

(1)The most recent Pension Protection Act zone status available in 2022 and 2021 is for the plan's year-end December 31, 2021 and December 31, 2020, 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.