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Pension and Other Postretirement Benefits (Tables) | 12 Months Ended
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Dec. 31, 2020
Retirement Benefits [Abstract] |
Schedule of Changes in Projected Benefit Obligations | The change in benefit obligations and change in plan assets as of December 31 are as follows:
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2020 |  | 2019
Change in benefit obligation: |  |  |  |  |  |  |
Benefit obligation at beginning of year | $ | 358.4 |  |  | $ | 314.4 |  |  | $ | 107.7 |  |  | $ | 108.7 |
Service cost | 4.9 |  |  | 4.0 |  |  | 0.2 |  |  | 0.2 |
Interest cost | 11.4 |  |  | 13.3 |  |  | 3.1 |  |  | 4.5 |
Actuarial (gain) loss | 31.1 |  |  | 46.5 |  |  | 2.7 |  |  | 0.1 |
Medicare Part D | — |  |  | — |  |  | 0.2 |  |  | 0.3 |
Benefits paid | (20.0) |  |  | (19.8) |  |  | (6.0) |  |  | (6.1) |
Benefit obligation at end of year | $ | 385.8 |  |  | $ | 358.4 |  |  | $ | 107.9 |  |  | $ | 107.7 |
Schedule of Changes in Fair Value of Plan Assets |
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2020 |  | 2019
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at beginning of year | $ | 295.8 |  |  | $ | 261.7 |  |  | $ | — |  |  | $ | — |
Actual return on plan assets | 41.1 |  |  | 52.2 |  |  | — |  |  | — |
Employer contributions | 1.9 |  |  | 1.8 |  |  | 5.8 |  |  | 5.8 |
Medicare Part D subsidy received | — |  |  | — |  |  | 0.2 |  |  | 0.3 |
Benefits paid | (20.0) |  |  | (19.8) |  |  | (6.0) |  |  | (6.1) |
Fair value of assets at end of year | $ | 318.8 |  |  | $ | 295.9 |  |  | $ | — |  |  | $ | — |
Schedule of Amounts Recognized in Balance Sheet |
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2020 |  | 2019
Funded status of plans: |  |  |  |  |  |  |
Funded status | $ | (67.0) |  |  | $ | (62.6) |  |  | $ | (107.9) |  |  | $ | (107.7) |
Amounts recognized in the Consolidated Balance Sheets: |  |  |  |  |  |  |
Non-current assets | — |  |  | — |  |  | — |  |  | — |
Current liabilities | (1.8) |  |  | (1.8) |  |  | (6.4) |  |  | (7.1) |
Non-current liabilities | (65.2) |  |  | (60.8) |  |  | (101.5) |  |  | (100.7) |
Net amount recognized | $ | (67.0) |  |  | $ | (62.6) |  |  | $ | (107.9) |  |  | $ | (107.8) |
Amounts recognized in accumulated other comprehensive loss (pre-tax): |  |  |  |  |  |  |
Net loss | $ | 94.0 |  |  | $ | 89.7 |  |  | $ | 42.2 |  |  | $ | 41.7 |
Prior service cost (benefit) | 1.1 |  |  | 1.2 |  |  | (13.5) |  |  | (16.7) |
Total | $ | 95.1 |  |  | $ | 90.9 |  |  | $ | 28.7 |  |  | $ | 25.0 |
Schedule of Net Benefit Cost | Net Periodic Benefit Cost:
| Year Ended December 31,
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2018 |  | 2020 |  | 2019 |  | 2018
Service cost | $ | 4.9 |  |  | $ | 4.0 |  |  | $ | 4.3 |  |  | $ | 0.2 |  |  | $ | 0.2 |  |  | $ | 0.2 |
Interest cost | 11.4 |  |  | 13.3 |  |  | 12.4 |  |  | 3.1 |  |  | 4.5 |  |  | 4.2 |
Expected return on plan assets | (20.8) |  |  | (18.3) |  |  | (21.1) |  |  | — |  |  | — |  |  | — |
Amortization of prior service costs | 0.1 |  |  | 0.1 |  |  | 0.1 |  |  | (3.2) |  |  | (4.9) |  |  | (7.3) |
Amortization of net loss | 6.5 |  |  | 6.6 |  |  | 5.2 |  |  | 2.2 |  |  | 2.3 |  |  | 4.0 |
Net periodic benefit cost | $ | 2.1 |  |  | $ | 5.7 |  |  | $ | 0.9 |  |  | $ | 2.3 |  |  | $ | 2.1 |  |  | $ | 1.2 |
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,
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2020 |  | 2019
Net loss (gain) | $ | 10.8 |  |  | $ | 12.6 |  |  | $ | 2.7 |  |  | $ | 0.1 |
Prior service cost (benefit) | — |  |  | — |  |  | — |  |  | — |
Amortization of net loss, including recognition due to settlement | (6.5) |  |  | (6.6) |  |  | (2.2) |  |  | (2.3) |
Amortization of prior service (cost) benefit, including recognition due to curtailment | (0.1) |  |  | (0.1) |  |  | 3.2 |  |  | 4.9 |
Total amount recognized in other comprehensive loss | 4.2 |  |  | 5.9 |  |  | 3.7 |  |  | 2.7 |
Net periodic benefit cost | 2.1 |  |  | 5.7 |  |  | 2.3 |  |  | 2.1 |
Total recognized in net periodic benefit cost and other comprehensive loss | $ | 6.3 |  |  | $ | 11.6 |  |  | $ | 6.0 |  |  | $ | 4.8 |
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:
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2020 |  | 2019
Discount rate (1) | 2.58% |  | 3.26% |  | 2.34% |  | 3.07%
Rate of compensation increase (2) | 3%/3.5% |  | 3%/3.5% |  | 3%/3.5% |  | 3%/3.5%
Measurement date | 12/31/2020 |  | 12/31/2019 |  | 12/31/2020 |  | 12/31/2019
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
| Pension |  | OPEB
| 2020 |  | 2019 |  | 2018 |  | 2020 |  | 2019 |  | 2018
Measurement date | 12/31/2019 |  | 12/31/2018 |  | 12/31/2017 |  | 12/31/2019 |  | 12/31/2018 |  | 12/31/2017
Fiscal year end | 12/31/2020 |  | 12/31/2019 |  | 12/31/2018 |  | 12/31/2020 |  | 12/31/2019 |  | 12/31/2018
Discount rate (1) | 3.22% |  | 4.38% |  | 3.69% |  | 2.79% |  | 4.29% |  | 3.66%
Rate of compensation increase (3) | 3%/3.5% |  | 3%/3.5% |  | 3%/4% |  | 3%/3.5% |  | 3%/3.5% |  | 3%/4%
Expected return on plan assets (4) | 7.25% |  | 7.25% |  | 7.18% |  | — |  | — |  | —
(1)We use the Ryan Above Median Yield Curve to determine the discount rate. (2)For 2020 and 2019, the rate of compensation increase is 3% per year for the first year and 3.5% per year thereafter. (3)For 2020, the rate of compensation increase is 3% per year for the first year and 3.5% per year thereafter. For 2019, the rate of compensation increase is 3% for the first year and 3.5% per year thereafter. For 2018, the rate of compensation increase is 3% per year for the first two years and 4% per year thereafter. (4)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
| 2020 Target |  | December 31, 2020 |  | December 31, 2019
Equities: |  |  |  |  |
U.S. equities | 30% |  | 31% |  | 27%
International equities | 26% |  | 27% |  | 23%
Fixed income | 44% |  | 42% |  | 50%
|  |  | 100% |  | 100%
Fair Value of Pension Plans’ assets included under the fair value hierarchy:
As of December 31, 2020 | Level 1 |  | Level 2 |  | Level 3 |  | Total
Equities: |  |  |  |  |  |  |
U.S. equities | $ | 98.4 |  |  | $ | — |  |  | $ | — |  |  | $ | 98.4 |
International equities | 86.8 |  |  | — |  |  | — |  |  | 86.8 |
Fixed income | 133.6 |  |  | — |  |  | — |  |  | 133.6 |
Total | $ | 318.8 |  |  | $ | — |  |  | $ | — |  |  | $ | 318.8 |
As of December 31, 2019 |  |  |  |  |  |  |
Equities: |  |  |  |  |  |  |
U.S. equities | $ | 78.5 |  |  | $ | — |  |  | $ | — |  |  | $ | 78.5 |
International equities | 66.7 |  |  | — |  |  | — |  |  | 66.7 |
Fixed income | 150.7 |  |  | — |  |  | — |  |  | 150.7 |
Total | $ | 295.9 |  |  | $ | — |  |  | $ | — |  |  | $ | 295.9 |
Schedule Of Expected Benefit Plan Contributions | We expect to make the following contributions for 2021:
| 2021
Expected pension plan contributions | $ | 1.8 |
Expected OPEB benefits payments | 6.4 |
Schedule of Expected Benefit Payments | The following table provides the estimated future benefit payments for the pension and other postretirement benefit plans:
| Pension Benefits |  | OPEB Benefits
2021 | $ | 20.8 |  |  | $ | 6.4 |
2022 | 20.9 |  |  | 6.4 |
2023 | 20.6 |  |  | 6.3 |
2024 | 20.3 |  |  | 6.3 |
2025 | 20.2 |  |  | 6.2 |
2026 – 2030 | 99.7 |  |  | 30.1 |
Schedule of Multiemployer Plans | Century’s participation in the plan for the year ended December 31, 2020, is outlined in the table below.
Fund |  | Steelworkers Pension Trust
EIN / PN |  | 23-6648508/499
Pension Protection Act Zone Status 2020 (1) |  | Green
Pension Protection Act Zone Status 2019 (1) |  | Green
Subject to Financial Improvement/Rehabilitation Plan (2) |  | No
Contributions of Century Aluminum 2020 |  | $1.4
Contributions of Century Aluminum 2019 |  | $1.3
Contributions of Century Aluminum 2018 |  | $1.0
Withdrawal from Plan Probable |  | No
Surcharge Imposed |  | No
Expiration Date of Collective Bargaining Agreement (2) |  | March 31, 2021
(1)The most recent Pension Protection Act zone status available in 2020 and 2019 is for the plan's year-end December 31, 2019 and December 31, 2018, 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.

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