Significant Accounting Policies (Policies)	6 Months Ended
Significant Accounting Policies (Policies)	Jun. 30, 2020
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 Securities and Exchange Commission (“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 on Form 10‑K for the year ended December 31, 2019. 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. Certain reclassifications have been made to the prior period’s consolidated financial statements and related footnotes to conform them to the current period presentation. Intercompany balances and transactions between consolidated entities are 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 $1.4 million at June 30, 2020 and $1.3 million at December 31, 2019, 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. 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 June 30, 2020, the estimated aggregate liability for uninsured claims totaled $1.2 million. Of this, $0.9 million is included in other long-term liabilities within the consolidated balance sheets. At December 31, 2019, the estimated aggregate liability for uninsured claims totaled $1.0 million including $0.7 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.
Deferred income	Deferred Income—We account for the SBA Paycheck Protection Program Loan (“PPP Loan”) as an in-substance government grant because we expect to meet the PPP Loan eligibility criteria and have concluded that the loan represents, in substance, a grant that is expected to be forgiven. Proceeds from the PPP Loan have been initially recognized as a deferred income liability. Subsequently, we reduce this liability and recognize income on a systematic basis over the period in which the related costs for which the PPP Loan is intended are incurred. PPP Loan income is presented as other income within the consolidated statements of operations.
Financial Instruments	Financial Instruments—Our financial assets and liabilities consist of cash, accounts receivable, accounts payable and notes payable. 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 June 30, 2020, sales to four customers accounted for approximately 26%, 16%, 16% and 15% of our total revenue, respectively, aggregating approximately 73% of our total revenue. During the six months ended June 30, 2020, sales to two customers accounted for approximately 33% and 17% of our total revenue, respectively, aggregating approximately 50% of our total revenue. The balance due from these two customers at June 30, 2020 was approximately 45% of total accounts receivable. During the three and six months ended June 30, 2019, sales to four customers accounted for approximately 55% of total revenue in each period.
Recent Accounting Pronouncements	Recent Accounting Pronouncements—In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses, which replaces the existing incurred loss impairment model with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. We adopted this standard effective January 1, 2020. The adoption of this ASU did not have a material impact on our consolidated financial statements. In August 2018, the FASB issued ASU 2018-15, Internal-Use Software, which addresses the accounting for implementation costs associated with a hosted service. The standard provides that implementation costs be evaluated for capitalization using the same criteria as that used for internal-use software development costs, with amortization expense being recorded in the same income statement expense line as the hosted service costs and over the expected term of the hosting arrangement. We adopted this standard as of January 1, 2020 on a prospective basis. The adoption of this ASU did not have a material impact on our consolidated financial statements. 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 will be effective for us in the first quarter of our fiscal year 2021. We do not expect that the adoption of this ASU will have a significant impact on our consolidated financial statements.