Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2019
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, 2018. 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 at September 30, 2019 were $1.2 million, 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 September 30, 2019, the estimated aggregate liability for uninsured claims totaled $0.9 million. Of this, $0.6 million is included in other long-term liabilities within the consolidated balance sheets. 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 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 and nine months ended September 30, 2019, sales to three customers accounted for approximately 58% and 49% of total revenue, respectively. The balance due from these three customers at September 30, 2019 was approximately 64% of total accounts receivable. During the three and nine months ended September 30, 2018 sales to six customers accounted for approximately 66% and 71% of total revenue, respectively.
Recent Accounting Pronouncements	Recent Accounting Pronouncements—In February 2016, the FASB issued ASU 2016‑02, Leases, which aims to make leasing activities more transparent and comparable and requires substantially all leases be recognized by lessees on their balance sheet as a right-of-use asset and corresponding lease liability, including leases currently accounted for as operating leases. Leases of mineral reserves and related land leases have been exempted from the standard. We adopted ASU 2016‑02, Leases, on January 1, 2019. We elected the “package of practical expedients” within the standard which permits us not to reassess prior conclusions about lease identification, lease classification and initial direct costs. We made an accounting policy election to not separate lease and non-lease components for all leases. The adoption of this standard resulted in the recognition of right-of-use assets and lease liabilities of $0.3 million, which were not previously recorded on our balance sheet.