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COMPONENTS OF PENSION PLAN NET PERIODIC BENEFIT COSTS | 6 Months Ended
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Jun. 30, 2021
COMPONENTS OF PENSION PLAN NET PERIODIC BENEFIT COSTS |
COMPONENTS OF PENSION PLAN NET PERIODIC BENEFIT COSTS | 15.COMPONENTS OF PENSION PLAN NET PERIODIC BENEFIT COSTS ​ Eligible employees at certain of our mining operations participate in a defined benefit plan (the "Pension Plan") that we sponsor. The Pension Plan is currently closed to new applicants and 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. Components of the net periodic benefit cost for each of the periods presented are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended | ​ | Six Months Ended | ​
​ | ​ | June 30, | ​ | June 30, | ​
​ | ​ | 2021 |  | 2020 |  | 2021 |  | 2020 |
​ | ​ | (in thousands) | ​
Interest cost | ​ | $ | 860 | ​ | $ | 1,054 | ​ | $ | 1,720 | ​ | $ | 2,108 | ​
Expected return on plan assets | ​ |  | (1,671) | ​ |  | (1,491) | ​ |  | (3,342) | ​ |  | (2,983) | ​
Amortization of prior service cost | ​ | ​ | 46 | ​ | ​ | 47 | ​ | ​ | 93 | ​ | ​ | 93 | ​
Amortization of net loss | ​ |  | 1,142 | ​ |  | 1,063 | ​ |  | 2,283 | ​ |  | 2,127 | ​
Net periodic benefit cost (1) | ​ | $ | 377 | ​ | $ | 673 | ​ | $ | 754 | ​ | $ | 1,345 | ​
| (1) | Net periodic benefit cost for the Pension Plan is included in the Other expense line item within our condensed consolidated statements of income.
​ During the six months ended June 30, 2021, we made contribution payments totaling $3.3 million to the Pension Plan for the 2020 plan year. As a result of certain pension plan contribution relief provided by the American Rescue Plan Act enacted in March 2021, we do not expect to make additional contributions to the Pension Plan during 2021 or 2022.

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