Significant Accounting Policies (Policies)	12 Months Ended
Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates: The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and judgments. These estimates and judgments affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities (if any) at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents: Cash and cash equivalents include cash in banks and highly liquid investments with original maturities of three months or less.
Inventories	Inventories: NACoal inventories are stated at the lower of cost or net realizable value. The weighted average method is used for inventory.
Property, Plant and Equipment, Net	Property, Plant and Equipment, Net: Property, plant and equipment are initially recorded at cost. Depreciation, depletion and amortization are provided in amounts sufficient to amortize the cost of the assets, including assets recorded under capital leases, over their estimated useful lives using the straight-line method. Buildings and building improvements are depreciated over the life of the mine, which is generally 30 years. Estimated lives for machinery and equipment range from three to 15 years. The units-of-production method is used to amortize certain coal-related assets based on estimated recoverable tonnages. Repairs and maintenance costs are generally expensed when incurred. Asset retirement costs associated with asset retirement obligations are capitalized with the carrying amount of the related long-lived asset and depreciated over the asset's estimated useful life.
Long-Lived Assets	Long-Lived Assets: The Company periodically evaluates long-lived assets for impairment when changes in circumstances or the occurrence of certain events indicate the carrying amount of an asset may not be recoverable. Upon identification of indicators of impairment, the Company evaluates the carrying value of the asset by comparing the estimated future undiscounted cash flows generated from the use of the asset and its eventual disposition with the asset's net carrying value. If the carrying value of an asset is considered impaired, an impairment charge is recorded for the amount that the carrying value of the long-lived asset exceeds its fair value. Fair value is estimated as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.
Coal Supply Agreement	Coal Supply Agreement: The coal supply agreement represents a long-term supply agreement with a NACoal customer and was recorded based on the fair value at the date of acquisition. The coal supply agreement is amortized based on units of production over the term of the agreement, which is estimated to be 30 years. The Company reviews identified intangible assets for impairment when changes in circumstances or the occurrence of certain events indicate potential impairment.
Self-insurance Liabilities	Self-insurance Liabilities: The Company is generally self-insured for medical claims, certain workers’ compensation claims and certain closed mine liabilities. An estimated provision for claims reported and for claims incurred but not yet reported under the self-insurance programs is recorded and revised periodically based on industry trends, historical experience and management judgment. In addition, industry trends are considered within management's judgment for valuing claims. Changes in assumptions for such matters as legal judgments and settlements, inflation rates, medical costs and actual experience could cause estimates to change in the near term.
Revenue Recognition	Revenue Recognition: Revenues are generally recognized when title transfers and risk of loss passes to the customer. Under its mining contracts, the Company recognizes revenue as the coal or limerock is delivered or services are performed.
Stock Compensation	Stock Compensation: The Company maintains long-term incentive programs. The parent company has stock compensation plans that allow the grant of shares of Class A common stock, subject to restrictions, as a means of retaining and rewarding selected employees for long-term performance and to increase ownership in the Company. Shares awarded under the plans are fully vested and entitle the stockholder to all rights of common stock ownership except that shares may not be assigned, pledged or otherwise transferred during the restriction period. In general, the restriction period ends at the earliest of (i) five years after the participant's retirement date, (ii) ten years from the award date, or (iii) the participant's death or permanent disability. Pursuant to the plans, the Company issued 92,572 and 62,425 shares related to the years ended December 31, 2017 and 2016, respectively. After the issuance of these shares, there were 407,428 shares of Class A common stock available for issuance under these plans. Compensation expense related to these share awards was $3.5 million ($2.3 million net of tax), $4.3 million ($2.8 million net of tax) and $1.7 million ($1.1 million net of tax) for the years ended December 31, 2017, 2016 and 2015, respectively. Compensation expense represents fair value based on the market price of the shares of Class A common stock at the grant date. The Company also has a stock compensation plan for non-employee directors of the Company under which a portion of the annual retainer for each non-employee director is paid in restricted shares of Class A common stock. For the three months ended December 31, 2017, $22,500 of the non-employee director's annual retainer of $37,500 was paid in restricted shares of Class A common stock. For the nine months ended September 30, 2017, $66,750 of the non-employee director's annual retainer of $108,750 was paid in restricted shares of Class A common stock. For the year ended December 31, 2016, $82,000 of the non-employee director's annual retainer of $138,000 was paid in restricted shares of Class A common stock. For the year ended December 31, 2015, $75,000 of the non-employee director's annual retainer of $131,000 was paid in restricted shares of Class A common stock. Shares awarded under the plan are fully vested and entitle the stockholder to all rights of common stock ownership except that shares may not be assigned, pledged or otherwise transferred during the restriction period. In general, the restriction period ends at the earliest of (i) ten years from the award date, (ii) the date of the director's death or permanent disability, (iii) five years (or earlier with the approval of the Board of Directors) after the director's date of retirement from the Board of Directors, or (iv) the date the director has both retired from the Board of Directors and has reached age 70. Pursuant to this plan, the Company issued 18,643, 10,690 and 11,496 shares related to the years ended December 31, 2017, 2016 and 2015, respectively. In addition to the mandatory retainer fee received in restricted stock, directors may elect to receive shares of Class A common stock in lieu of cash for up to 100% of the balance of their annual retainer, committee retainer and any committee chairman's fees. These voluntary shares are not subject to any restrictions.
Financial Instruments and Derivative Financial Instruments	Financial Instruments and Derivative Financial Instruments: Financial instruments held by the Company include cash and cash equivalents, accounts receivable, accounts payable, revolving credit agreements, long-term debt and interest rate swap agreements. The Company uses interest rate swap agreements to partially reduce risks related to floating rate financing agreements that are subject to changes in the market rate of interest. Terms of the interest rate swap agreements require the Company to receive a variable interest rate and pay a fixed interest rate. The Company's interest rate swap agreements and its variable rate financings are predominately based upon LIBOR (London Interbank Offered Rate). The Company does not hold or issue financial instruments or derivative financial instruments for trading purposes. Beginning in 2017, gains and losses for the interest rate swap agreements held by the Company were no longer deferred in AOCI, but were recorded in the Consolidated Statement of Operations.
Fair Value Measurements	Fair Value Measurements: The Company accounts for the fair value measurement of its financial assets and liabilities in accordance with U.S. generally accepted accounting principles, which defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A fair value hierarchy requires an entity to maximize the use of observable inputs, where available, and minimize the use of unobservable inputs when measuring fair value. Described below are the three levels of inputs that may be used to measure fair value: Level 1 - Quoted prices in active markets that are accessible at the measurement date for identical assets or liabilities. Level 2 - Observable prices that are based on inputs not quoted on active markets, but corroborated by market data. Level 3 - Unobservable inputs are used when little or no market data is available. The hierarchy is based upon the transparency of inputs to the valuation of an asset or liability as of the measurement date. The classification of fair value measurements within the hierarchy is based upon the lowest level of input that is significant to the measurement.
Recently Issued Accounting Standards	Recently Issued Accounting Standards Accounting Standards Not Yet Adopted: In May 2014, the FASB codified ASC 606, "Revenue Recognition - Revenue from Contracts with Customers," which supersedes most current revenue recognition guidance, including industry-specific guidance, and requires an entity to recognize revenue in an amount that reflects the consideration to which the entity expects to be entitled in exchange for transferring goods or services to customers and provide additional disclosures. As amended, the effective date for public entities is annual reporting periods beginning after December 15, 2017 and interim periods therein. The Company adopted the new revenue guidance effective January 1, 2018 using the modified retrospective method with the cumulative effect of initially applying the standard recognized as an adjustment to equity. The Company developed a project plan with respect to its implementation of this standard, including identification of revenue streams and review of contracts and procedures currently in place. The Company completed its review of customer contracts at its NACoal subsidiary, including the contracts for the Company’s Unconsolidated Mines. While the revenue of the Unconsolidated Mines is not consolidated within the Company’s financial statements, any change in the amount or timing of revenue recognition at the Unconsolidated Mines could have an impact on the company’s recognition of earnings from the unconsolidated mines. During the fourth quarter of 2017, the Company completed its evaluation of ASC 606, including identifying and implementing changes to processes and controls to meet the standard's updated reporting and disclosure requirements, including the calculation of the cumulative effect of adopting ASC 606. There were no significant changes to accounting systems or controls upon adoption of ASC 606. The Company assessed the goods and services promised in its contracts with customers and identified a performance obligation for each promised good or service that is distinct. Each mine has a contract with its respective customer that represents a contract under ASC 606. For its consolidated entities, NACoal’s performance obligations vary by contract and consist of the following: At MLMC, each MMBtu delivered during the production period is considered a separate performance obligation. Revenue is recognized at the point in time that control of each MMBtus of lignite transfers to the customer. Fluctuations in revenue from period to period generally result from changes in customer demand. At NAM entities, the management service to oversee the maintenance and operation of the draglines and other mining equipment and delivery of limerock is the performance obligation, which is accounted for as a series. As each month of service is completed, revenue is recognized for the amount of actual costs incurred, plus the management fee and the general and administrative fee (as applicable). Fluctuations in revenue from period to period result from changes in customer demand and variances in reimbursable costs primarily associated with production levels on individual contracts. NACoal enters into royalty contracts which grant the lessee the right to explore, develop, produce and sell minerals controlled by the Company. These arrangements result in the transfer of mineral rights to the lessee for a period of time to permit access for purposes of exploration, development, and production. The mineral rights revert back to NACoal at the expiration of the contracts. Under these royalty contracts, NACoal’s grant to the lessee of rights to minerals is the performance obligation. The performance obligation under these contracts represents a series of distinct goods or services whereby each day of access that is provided is distinct. The transaction price consists of a variable sales based royalty and in certain arrangements a fixed component in the form of an up-front lease bonus payment. As the amount of consideration the Company will ultimately be entitled to is entirely susceptible to factors outside its control, the entire amount of variable consideration is constrained at contract inception. The fixed portion of the transaction price will be recognized over the initial term of the contract which is generally five years. The adoption of ASC 606 will result in the establishment of a $2.6 million contract liability and a $2.1 million cumulative effect adjustment to beginning retained earnings (net of tax of $0.5 million) as of January 1, 2018 to reflect the impact of changing the accounting for lease bonus payments received under certain royalty contracts. The Company does not expect any other changes to the timing of revenue recognition under ASC 606. The adoption of this guidance will result in increased disclosures to help users of financial statements understand the nature, amount and timing of revenue and cash flows arising from contracts. In January 2016, the FASB issued ASU No. 2016-01, "Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities," which modifies how entities measure equity investments and present changes in the fair value of financial liabilities; simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment; changes presentation and disclosure requirements; and clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s other deferred tax assets. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The adoption of this guidance will not have a material effect on the Company’s financial position, results of operations, cash flows and related disclosures. In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842)," which requires an entity to recognize assets and liabilities for the rights and obligations created by leased assets. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company is currently evaluating how and to what extent ASU 2016-02 will affect the Company's financial position, results of operations, cash flows and related disclosures.
Reclassifications	Reclassifications: As a result of the spin-off and the reclassification of HBBHC to discontinued operations, certain amounts in the prior periods’ Consolidated Financial Statements have been reclassified to conform to the current period's presentation.