Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation—These interim financial statements are unaudited and have been prepared pursuant to the rules and regulations of the SEC regarding interim financial reporting. Certain disclosures have been condensed or omitted from these financial statements. Accordingly, they do not include all the information and notes required by accounting principles generally accepted in the United States of America (“GAAP”) for complete consolidated financial statements, and should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report. In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring adjustments, necessary to fairly present the financial position as of, and the results of operations for, all periods presented. In preparing the accompanying financial statements, management has made certain estimates and assumptions that affect reported amounts in the condensed consolidated financial statements and disclosures of contingencies. Actual results may differ from those estimates. The results for interim periods are not necessarily indicative of annual results. Intercompany balances and transactions between consolidated entities have been eliminated.
Cash and cash equivalents	Cash and Cash Equivalents—We classify all highly-liquid instruments with an original maturity of three months or less to be cash equivalents. Restricted cash balances were $1.0 million at September 30, 2021 and $1.4 million at December 31, 2020. These consisted of funds held in escrow for potential future workers’ compensation claims and were classified in other current assets in the consolidated balance sheets.
Self-Insurance	Self-Insurance—We are self-insured for certain losses relating to workers’ compensation claims, including pneumoconiosis (occupational disease) claims. We purchase insurance coverage to reduce our exposure to significant levels of these claims. Self-insured losses are accrued based upon estimates of the aggregate liability for uninsured claims incurred as of the balance sheet date using current and historical claims experience and certain actuarial assumptions. At September 30, 2021, the estimated aggregate liability for uninsured claims totaled $3.5 million. Of this, $2.1 million is included in other long-term liabilities within the consolidated balance sheets. At December 31, 2020, the estimated aggregate liability for uninsured claims totaled $1.7 million including $0.9 million included in other long-term liabilities. These estimates are subject to uncertainty due to a variety of factors, including extended lag times in the reporting and resolution of claims, and trends or changes in claim settlement patterns, insurance industry practices and legal interpretations. As a result, actual costs could differ significantly from the estimated amounts. Adjustments to estimated liabilities are recorded in the period in which the change in estimate occurs.
Financial Instruments	Financial Instruments—Our financial assets and liabilities consist of cash, accounts receivable, accounts payable and indebtedness. The fair values of these instruments approximate their carrying amounts at each reporting date. Nonrecurring fair value measurements include asset retirement obligations, the estimated fair value of which is calculated as the present value of estimated cash flows related to its reclamation liabilities using Level 3 inputs. The significant inputs used to calculate such liabilities include estimates of costs to be incurred, our credit adjusted discount rate, inflation rates and estimated date of reclamation.
Concentrations	Concentrations—During the three months ended September 30, 2021, sales to our top three customers accounted for approximately 29%, 19% and 16% of our total revenue, respectively, aggregating to approximately 64% of our total revenue. The balances due in the aggregate from these three customers at September 30, 2021 was approximately 63% of our total accounts receivable. During the nine months ended September 30, 2021, sales to our top three customers accounted for approximately 31%, 18% and 10% of our total revenue, respectively, aggregating to approximately 59% of our total revenue. The balances due in the aggregate from these three customers at September 30, 2021 was approximately 63% of our total accounts receivable. During the three months ended September 30, 2020, sales to our top three customers accounted for approximately 86% of total revenue. During the nine months ended September 30, 2020, sales to our top three customers accounted for approximately 71% of total revenue.
Recent Accounting Pronouncements	Recent Accounting Pronouncements—In December 2019, the FASB issued ASU 2019-12, Income Taxes, which enhances and simplifies various aspects of the income tax accounting guidance, including requirements such as tax basis step-up in goodwill obtained in a transaction that is not a business combination, ownership changes in investments, and interim-period accounting for enacted changes in tax law. The standard was effective for us in the first quarter of our fiscal year 2021. The adoption of this ASU did not have a material impact on our consolidated financial statements. In March 2020, the FASB issued ASU No. 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting, which provides optional expedients and exceptions for applying generally accepted accounting principles to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments are effective for all entities beginning on March 12, 2020 through December 31, 2022. The Company has not adopted this ASU. The Company is currently assessing the impact of adopting this standard on its consolidated financial statements and the timing of adoption.

Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2021

Basis of Presentation—These interim financial statements are unaudited and have been prepared pursuant to the rules and regulations of the SEC regarding interim financial reporting. Certain disclosures have been condensed or omitted from these financial statements. Accordingly, they do not include all the information and notes required by accounting principles generally accepted in the United States of America (“GAAP”) for complete consolidated financial statements, and should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report.

In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring adjustments, necessary to fairly present the financial position as of, and the results of operations for, all periods presented. In preparing the accompanying financial statements, management has made certain estimates and assumptions that affect reported amounts in the condensed consolidated financial statements and disclosures of contingencies. Actual results may differ from those estimates. The results for interim periods are not necessarily indicative of annual results. Intercompany balances and transactions between consolidated entities have been eliminated.

Cash and cash equivalents

Cash and Cash Equivalents—We classify all highly-liquid instruments with an original maturity of three months or less to be cash equivalents. Restricted cash balances were $1.0 million at September 30, 2021 and $1.4 million at December 31, 2020. These consisted of funds held in escrow for potential future workers’ compensation claims and were classified in other current assets in the consolidated balance sheets.

Self-Insurance

Self-Insurance—We are self-insured for certain losses relating to workers’ compensation claims, including pneumoconiosis (occupational disease) claims. We purchase insurance coverage to reduce our exposure to significant levels of these claims. Self-insured losses are accrued based upon estimates of the aggregate liability for uninsured claims incurred as of the balance sheet date using current and historical claims experience and certain actuarial assumptions. At September 30, 2021, the estimated aggregate liability for uninsured claims totaled $3.5 million. Of this, $2.1 million is included in other long-term liabilities within the consolidated balance sheets. At December 31, 2020, the estimated aggregate liability for uninsured claims totaled $1.7 million including $0.9 million included in other long-term liabilities. These estimates are subject to uncertainty due to a variety of factors, including extended lag times in the reporting and resolution of claims, and trends or changes in claim settlement patterns, insurance industry practices and legal interpretations. As a result, actual costs could differ significantly from the estimated amounts. Adjustments to estimated liabilities are recorded in the period in which the change in estimate occurs.

Financial Instruments

Financial Instruments—Our financial assets and liabilities consist of cash, accounts receivable, accounts payable and indebtedness. The fair values of these instruments approximate their carrying amounts at each reporting date.

Nonrecurring fair value measurements include asset retirement obligations, the estimated fair value of which is calculated as the present value of estimated cash flows related to its reclamation liabilities using Level 3 inputs. The significant inputs used to calculate such liabilities include estimates of costs to be incurred, our credit adjusted discount rate, inflation rates and estimated date of reclamation.

Concentrations

Concentrations—During the three months ended September 30, 2021, sales to our top three customers accounted for approximately 29%, 19% and 16% of our total revenue, respectively, aggregating to approximately 64% of our total revenue. The balances due in the aggregate from these three customers at September 30, 2021 was approximately 63% of our total accounts receivable. During the nine months ended September 30, 2021, sales to our top three customers accounted for approximately 31%, 18% and 10% of our total revenue, respectively, aggregating to approximately 59% of our total revenue. The balances due in the aggregate from these three customers at September 30, 2021 was approximately 63% of our total accounts receivable. During the three months ended September 30, 2020, sales to our top three customers accounted for approximately 86% of total revenue. During the nine months ended September 30, 2020, sales to our top three customers accounted for approximately 71% of total revenue.

Recent Accounting Pronouncements

Recent Accounting Pronouncements—In December 2019, the FASB issued ASU 2019-12, Income Taxes, which enhances and simplifies various aspects of the income tax accounting guidance, including requirements such as tax basis step-up in goodwill obtained in a transaction that is not a business combination, ownership changes in investments, and interim-period accounting for enacted changes in tax law. The standard was effective for us in the first quarter of our fiscal year 2021. The adoption of this ASU did not have a material impact on our consolidated financial statements.

In March 2020, the FASB issued ASU No. 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting, which provides optional expedients and exceptions for applying generally accepted accounting principles to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments are effective for all entities beginning on March 12, 2020 through December 31, 2022. The Company has not adopted this ASU. The Company is currently assessing the impact of adopting this standard on its consolidated financial statements and the timing of adoption.