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ASSET RETIREMENT OBLIGATIONS | 12 Months Ended
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Dec. 31, 2019
ASSET RETIREMENT OBLIGATIONS |
ASSET RETIREMENT OBLIGATIONS | 18.ASSET RETIREMENT OBLIGATIONS ​ The majority of our operations are governed by various state statutes and the Federal Surface Mining Control and Reclamation Act of 1977, which establish reclamation and mine closing standards. These regulations require, among other things, restoration of property in accordance with specified standards and an approved reclamation plan. ​ The following table presents the activity affecting the asset retirement and mine closing liability: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31, |
​ |  | 2019 |  | 2018 |
​ | ​ | (in thousands) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Beginning balance | ​ | $ | 137,114 | ​ | $ | 130,600 | ​
Accretion expense | ​ |  | 4,087 | ​ |  | 3,926 | ​
Payments | ​ |  | (2,948) | ​ |  | (2,392) | ​
Allocation of liability associated with acquisitions, mine development and change in assumptions | ​ |  | (739) | ​ |  | 4,980 | ​
Ending balance | ​ | $ | 137,514 | ​ | $ | 137,114 | ​
​ For the year ended December 31, 2019, the allocation of liability associated with acquisition, mine development and change in assumptions was immaterial. ​ For the year ended December 31, 2018, the allocation of liability associated with acquisition, mine development and change in assumptions was a net increase of $5.0 million. This net increase was attributable to the expansion of refuse sites primarily at the Hamilton County Coal, LLC ("Hamilton") and Tunnel Ridge, LLC ("Tunnel Ridge") mines, partially offset by decreased cost estimates for water related treatment at the Mettiki mine and completion of certain reclamation obligations at the Hopkins County Coal mining complex. ​ The impact of discounting our estimated cash flows resulted in reducing the accrual for asset retirement obligations by $102.9 million and $100.3 million at December 31, 2019 and 2018, respectively. Estimated payments of asset retirement obligations as of December 31, 2019 are as follows: ​
​ | ​ | ​ | ​ | ​
Year Ended | ​ | ​ | ​ | ​
December 31, |  | (in thousands) |
​ | ​ | ​ | ​ | ​
2020 | ​ | $ | 4,496 | ​
2021 | ​ |  | 2,576 | ​
2022 | ​ |  | 2,794 | ​
2023 | ​ |  | 2,027 | ​
2024 | ​ |  | 2,986 | ​
Thereafter | ​ |  | 225,584 | ​
Aggregate undiscounted asset retirement obligations | ​ |  | 240,463 | ​
Effect of discounting | ​ |  | (102,949) | ​
Total asset retirement obligations | ​ |  | 137,514 | ​
Less: current portion | ​ |  | (4,496) | ​
Asset retirement obligations | ​ | $ | 133,018 | ​
​ Federal and state laws require bonds to secure our obligations to reclaim lands used for mining and are typically renewable on a yearly basis. As of December 31, 2019 and 2018, we had approximately $181.6 million and $169.3 million, respectively, in surety bonds outstanding to secure the performance of our reclamation obligations. ​ See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for asset retirement obligations.

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- Definition The entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -URI http://asc.fasb.org/subtopic&trid=2175671 + Details
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