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EMPLOYEE BENEFIT PLANS | 12 Months Ended
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Dec. 31, 2023
COMPONENTS OF PENSION PLAN NET PERIODIC BENEFIT COSTS |
EMPLOYEE BENEFIT PLANS | 15.EMPLOYEE BENEFIT PLANS ​ Defined Contribution Plans ​ All regular full-time employees are eligible to participate in a defined contribution profit sharing and savings plan ("PSSP") that we sponsor. PSSP participants may elect to make voluntary contributions to this plan up to a specified amount of their compensation. We make matching contributions based on a percent of an employee's eligible compensation and also make an additional non-matching contribution. Our contribution expense for the PSSP was approximately $21.8 million, $19.4 million and $17.7 million for the years ended December 31, 2023, 2022 and 2021, respectively. ​ Defined Benefit Plan ​ Eligible employees and former employees of certain of our mining operations participate in a defined benefit plan (the "Pension Plan") that we sponsor. The Pension Plan is closed to new applicants. Participants in the Pension Plan are no longer receiving benefit accruals for service. The benefit formula for the Pension Plan is a fixed-dollar unit based on years of service. ​ The following sets forth changes in benefit obligations and plan assets for the years ended December 31, 2023 and 2022 and the funded status of the Pension Plan reconciled with the amounts reported in our consolidated financial statements: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, | ​
​ | ​ | 2023 |  | 2022 |
​ | ​ | (dollars in thousands) | ​
Change in benefit obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Benefit obligations at beginning of year | ​ | $ | 104,682 | ​ | $ | 139,566 | ​
Interest cost | ​ |  | 5,180 | ​ |  | 3,749 | ​
Actuarial loss (gain) | ​ |  | 2,446 | ​ |  | (32,996) | ​
Benefits paid | ​ |  | (6,438) | ​ |  | (5,637) | ​
Benefit obligations at end of year | ​ |  | 105,870 | ​ |  | 104,682 | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Change in plan assets: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Fair value of plan assets at beginning of year | ​ |  | 92,129 | ​ |  | 113,976 | ​
Actual return on plan assets | ​ |  | 11,561 | ​ |  | (16,210) | ​
Benefits paid | ​ |  | (6,438) | ​ |  | (5,637) | ​
Fair value of plan assets at end of year | ​ |  | 97,252 | ​ |  | 92,129 | ​
Funded status at the end of year | ​ | $ | (8,618) | ​ | $ | (12,553) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Amounts recognized in balance sheet: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Non-current liability | ​ | $ | (8,618) | ​ | $ | (12,553) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Amounts recognized in accumulated other comprehensive income consists of: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Prior service cost | ​ | $ | (196) | ​ | $ | (382) | ​
Net actuarial loss | ​ | ​ | (11,584) | ​ | ​ | (15,160) | ​
​ | ​ | $ | (11,780) | ​ | $ | (15,542) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Weighted-average assumption to determine benefit obligations as of December 31, | ​ | ​ | ​ | ​ | ​ | ​ | ​
Discount rate | ​ |  | 4.90% | ​ |  | 5.10% | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Weighted-average assumptions used to determine net periodic benefit cost for the year ended December 31, | ​ | ​ | ​ | ​ | ​ | ​ | ​
Discount rate | ​ |  | 5.10% | ​ |  | 2.73% | ​
Expected return on plan assets | ​ |  | 7.00% | ​ |  | 6.00% | ​
​ The actuarial loss component of the change in benefit obligations in 2023 was primarily attributable to a decrease in the discount rate compared to the prior year end. The actuarial gain component of the change in benefit obligations in 2022 was primarily attributable to an increase in the discount rate compared to the prior year end. ​ The expected long-term rate of return used to determine our pension liability is based on an asset allocation assumption of: ​
​ | ​ | ​ | ​
​ | ​ | ​ | ​
​ | ​ | Asset allocation | ​
As of December 31, 2023 |  | assumption |
Equity securities | ​ | 75% | ​
Fixed income securities |  | 25% | ​
​ |  | 100% | ​
​ The actual return on plan assets was 12.5% and (14.6)% for the years ended December 31, 2023 and 2022, respectively. ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31, |
​ |  | 2023 |  | 2022 |  | 2021 | ​
​ | ​ | (in thousands) |
Components of net periodic benefit cost (credit): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Interest cost | ​ | $ | 5,180 | ​ | $ | 3,749 | ​ | $ | 3,438 | ​
Expected return on plan assets | ​ |  | (6,220) | ​ |  | (6,638) | ​ |  | (6,580) | ​
Amortization of prior service cost | ​ | ​ | 186 | ​ | ​ | 186 | ​ | ​ | 186 | ​
Amortization of net loss | ​ |  | 682 | ​ |  | 1,963 | ​ |  | 4,327 | ​
Net periodic benefit cost (credit) (1) | ​ | $ | (172) | ​ | $ | (740) | ​ | $ | 1,371 | ​
(1) | Nonservice components of net periodic benefit cost (credit) are included in the Other income (expense) line item within our consolidated statements of income.
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Year Ended December 31,
​ | ​ | 2023 |  | 2022 | ​
​ | ​ | (in thousands) | ​
Other changes in plan assets and benefit obligation recognized in accumulated other comprehensive loss: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Net actuarial gain | ​ | $ | 2,894 | ​ | $ | 10,148 | ​
Reversal of amortization item: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Prior service cost | ​ | ​ | 186 | ​ | ​ | 186 | ​
Net actuarial loss | ​ |  | 682 | ​ |  | 1,963 | ​
Total recognized in accumulated other comprehensive loss | ​ |  | 3,762 | ​ |  | 12,297 | ​
Net periodic benefit cost | ​ |  | 172 | ​ |  | 740 | ​
Total recognized in net periodic benefit cost and accumulated other comprehensive loss | ​ | $ | 3,934 | ​ | $ | 13,037 | ​
​ Estimated future benefit payments as of December 31, 2023 are as follows: ​
​ | ​ | ​ | ​ | ​
Year Ended | ​ | ​ | ​ | ​
December 31, |  | (in thousands) |
​ | ​ | ​ | ​ | ​
2024 | ​ | $ | 6,331 | ​
2025 | ​ |  | 6,490 | ​
2026 | ​ |  | 6,688 | ​
2027 | ​ |  | 6,820 | ​
2028 | ​ |  | 6,899 | ​
2029-2033 | ​ |  | 35,204 | ​
​ | ​ | $ | 68,432 | ​
​ We do not expect to make material contributions to the Pension Plan during 2024. ​ The Compensation Committee has appointed an investment manager with full investment authority with respect to Pension Plan investments subject to investment guidelines and compliance with Employee Retirement Income Security Act of 1974 or other applicable laws. The investment manager employs an asset allocation strategy through investment in certain investment types such as equity securities and fixed income securities. The asset allocation process provides that the total portfolio allocation will be adjusted as the funded ratio of the plan changes and market conditions warrant, consistent with managing risks in accordance with plan objectives and time horizon. As the funded ratio improves, more assets may be allocated to the core fixed income portfolio to reduce volatility. The objective of the allocation policy is to achieve an average annual return greater than the actuarial discount rate over the specified time horizon. General asset allocation guidelines at December 31, 2023 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Percentage of Total Portfolio |
​ |  | Minimum |  | Maximum |
​ | ​ | ​ | ​ | ​ | ​
Equity securities | ​ | 50% | ​ | 85% | ​
Fixed income securities | ​ | 15% | ​ | 50% | ​
​ Equity securities include domestic and international common stocks, convertible notes and bonds, convertible preferred stocks, American Depository Receipts of non-U.S. companies and Real Estate Investment Trusts. Fixed income securities include debt securities issued by the federal government as well as state and local governments, banker's acceptances, repurchase agreements, asset-backed securities, collateralized mortgage-backed securities, corporate debt securities, inflation-index bonds and structured notes. ​ The following information discloses the fair values of our Pension Plan assets by asset category: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, |
​ | ​ | 2023 | ​ | 2022 | ​
​ | ​ | Level 1 | ​ | Level 2 | ​ | Level 3 | ​ | Total | ​ | Level 1 | ​ | Level 2 | ​ | Level 3 | ​ | Total | ​
​ | ​ | (in thousands) |
Cash and cash equivalents | ​ | $ | 1,665 | ​ | $ | — | ​ | $ | — | ​ | $ | 1,665 | ​ | $ | 5,422 | ​ | $ | — | ​ | $ | — | ​ | $ | 5,422 | ​
Equity investments - Individual securities (a): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Consumer discretionary | ​ | ​ | 2,605 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 2,605 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Consumer durables | ​ | ​ | 2,088 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 2,088 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Energy | ​ | ​ | 952 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 952 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Financials | ​ | ​ | 4,852 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 4,852 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Health Care | ​ | ​ | 4,296 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 4,296 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Industrials & materials | ​ | ​ | 4,507 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 4,507 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Information technology & communication | ​ | ​ | 8,102 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 8,102 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Fixed income investments - Individual securities (b): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Preferred stocks non-convertible | ​ | ​ | 37 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 37 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Equity investments - Mutual funds (c): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Mid-cap stock funds | ​ | ​ | 11,847 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 11,847 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Small-cap stock funds | ​ | ​ | 4,007 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 4,007 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
International stock funds | ​ | ​ | 7,849 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 7,849 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Equity investments - Exchange traded funds (d): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Large-cap blend - S&P 500 index | ​ | ​ | 18,044 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 18,044 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
International - Developed markets | ​ | ​ | 3,138 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 3,138 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
International - Emerging markets | ​ | ​ | 1,894 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 1,894 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
Accrued income (e) | ​ | ​ | — | ​ | ​ | 29 | ​ | ​ | — | ​ | ​ | 29 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​
​ | ​ | $ | 75,883 | ​ | $ | 29 | ​ | $ | — | ​ | $ | 75,912 | ​ | $ | 5,422 | ​ | $ | — | ​ | $ | — | ​ | $ | 5,422 | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commingled investment funds measured at net asset value (f): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Equities - United States | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 36,259 | ​
Equities - United States futures | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | (697) | ​
Equities - International developed markets | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 14,214 | ​
Equities - International developed markets futures | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | (1,693) | ​
Equities - International emerging markets | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 782 | ​
Equities - International emerging markets futures | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 3,289 | ​
Fixed income - Investment grade | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 21,340 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 13,856 | ​
Fixed income - High yield | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 156 | ​
Fixed income - Futures | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 8,590 | ​
Alternatives | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 11,951 | ​
Total | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 97,252 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | $ | 92,129 | ​
(a) | Equity investments - Individual securities include investments in publicly traded common stock and American Depository Receipts. Publicly traded common stocks are traded on a national securities exchange and investments in common stocks are valued using quoted market prices multiplied by the number of shares owned. American Depository Receipts are negotiable securities issued by a bank representing shares in a foreign company and traded on a national securities exchange.
(b) | Fixed income investments - Individual securities include investments in preferred stock that are traded on a national securities exchange and valued using quoted market prices multiplied by the number of shares owned.
(c) | Equity investments - Mutual funds are valued daily in actively traded markets. For purposes of calculating the value, portfolio securities and other assets for which market quotes are readily available are valued at market value. Investments initially valued in currencies other than the U.S. dollars are converted to the U.S. dollar using exchange rates obtained from pricing services.
(d) | Equity investments – Exchange traded funds are funds that own financial assets and trade on exchanges, generally tracking a specific index. Investments in exchange traded funds are valued using a market approach based on the quoted market prices.
(e) | Accrued income represents dividends or interest declared, but not received, on equity securities owned at December 31, 2023.
(f) | Investments measured at fair value using the net asset value per share (or its equivalent) have not been classified within the fair value hierarchy. The fair values of all commingled investment funds are determined based on the net asset values per unit of each of the funds. The net asset values per unit represent the aggregate value of the fund's assets at fair value less liabilities, divided by the number of units outstanding.

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