Entity Listings, Exchange [Axis] Entity Listings, Exchange [Axis] Award Timing Predetermined Award Timing Predetermined [Flag] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Cost reimbursement, commercial mining milestone, fee Commercial Mining Milestone Success Fee Commercial Mining Milestone Success Fee Entity File Number Entity File Number Capital in Excess of Par Value Additional Paid-in Capital [Member] Other Noncurrent Assets Other Noncurrent Assets [Member] Share of earnings Income (Loss) from Equity Method Investments Tabular List, Table Tabular List [Table Text Block] Earnings per share: Earnings Per Share, Basic [Abstract] Accounting Standards Not Yet Adopted New Accounting Pronouncements, Policy [Policy Text Block] Balance at January 1, 2026 Balance at March 31, 2026 Contract with Customer, Asset, after Allowance for Credit Loss, Current Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Measurement Basis [Axis] Measurement Basis [Axis] Basic earnings per share (in dollars per share) Earnings Per Share, Basic Adjustment to Compensation: Adjustment to Compensation [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Revenue Recognition Revenue from Contract with Customer [Text Block] Stock, Class of Stock [Table] Stock, Class of Stock [Table] Schedule of Segment Reporting Information Schedule of Segment Reporting Information, by Segment [Table Text Block] Inventory Disclosure [Abstract] Inventory Disclosure [Abstract] Mining supplies Other Inventory, Supplies, Gross Contract Mining Contract Mining [Member] North American Mining [Member] Revenue expected to be recognized Revenue, Remaining Performance Obligation, Amount Adjustment to Compensation, Amount Adjustment to Compensation Amount Affiliated Entity Affiliated Entity [Member] Employee Stock Option Share-Based Payment Arrangement, Option [Member] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Increase (decrease) in contract asset (long-term) Increase (Decrease) In Contract With Customer, Asset, After Allowance For Credit Loss, Noncurrent Increase (Decrease) In Contract With Customer, Asset, After Allowance For Credit Loss, Noncurrent Cash dividends on Class A and Class B common stock Dividends, Common Stock, Cash Total stockholders' equity Balance, beginning of period Balance, end of period Equity, Attributable to Parent Insider Trading Policies and Procedures [Line Items] Excess funding liability Excess Funding Liability Excess Funding Liability NYSE CHICAGO, INC. NYSE CHICAGO, INC. [Member] Increase (decrease) in trade accounts receivable Increase (decrease) in contract asset (current) Increase (Decrease) In Contract with Customer, Asset, after Allowance for Credit Loss, Current Increase (Decrease) In Contract with Customer, Asset, after Allowance for Credit Loss, Current Asset Total liabilities and equity Liabilities and Equity Accumulated Other Comprehensive Income (Loss) Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Cover [Abstract] Cover [Abstract] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Intangibles, net Finite-Lived Intangible Assets, Net Common stock, convertible conversion ratio Common Stock, Convertible Conversion Ratio Common Stock, Convertible Conversion Ratio Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Gain on sale of assets Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Pay vs Performance Disclosure [Line Items] Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Accrued payroll Accrued Salaries, Current Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Income before income tax provision Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Entity Shell Company Entity Shell Company Common stock: Common Stock, Number of Shares, Par Value and Other Disclosure [Abstract] Trade accounts receivable Change in Contract with Customer, Asset [Roll Forward] Change in Contract with Customer, Asset [Roll Forward] Other long-term liabilities Other Liabilities, Noncurrent Proceeds from the sale of assets Proceeds from Sale of Property, Plant, and Equipment Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Operating segments Operating Segments [Member] Variable Interest Entity [Table] Variable Interest Entity [Table] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Company Selected Measure Amount Company Selected Measure Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Interest income Investment Income, Nonoperating Name Measure Name Class of Stock [Line Items] Class of Stock [Line Items] Document Fiscal Period Focus Document Fiscal Period Focus Award Timing Method Award Timing Method [Text Block] Award Type Award Type [Axis] Balance at January 1, 2026 Balance at March 31, 2026 Contract with Customer, Liability, Noncurrent Legal Entity [Axis] Legal Entity [Axis] Total assets  Total assets Assets Capital contributions Equity Method Investments, Increase From Capital Contributions Equity Method Investments, Increase From Capital Contributions Investment, Name [Domain] Investment, Name [Domain] Trading Symbol Trading Symbol Entity Address, City or Town Entity Address, City or Town Operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Receivable Type [Axis] Receivable Type [Axis] Inventories Inventory Disclosure [Text Block] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Assets: Assets, Fair Value Disclosure [Abstract] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Entity Emerging Growth Company Entity Emerging Growth Company Cash dividends on common stock (in dollars per share) Common Stock, Dividends, Per Share, Declared Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Contract assets recognized Capitalized Contract Cost, Net Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Balance at March 31, 2026 Equity method investment in Eiger Resources Income from equity method investee Equity Method Investments Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding PEO PEO [Member] 2025 Stock Repurchase Program 2025 Stock Repurchase Program [Member] 2025 Stock Repurchase Program Schedule of Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Long-term contracts percentage Long-Term Contracts, Percentage Long-Term Contracts, Percentage Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Expenditures for property, plant and equipment and acquisition of mineral interests Expenditures for property, plant and equipment and acquisition of mineral interests Payments To Acquire Property, Plant, And Equipment And Mineral Rights Payments To Acquire Property, Plant, And Equipment And Mineral Rights Retained Earnings Retained Earnings [Member] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Restatement Determination Date Restatement Determination Date Balance at the beginning of the period Balance at the end of the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Income Statement [Abstract] Income Statement [Abstract] Income tax provision Income Tax Expense (Benefit) Shares Outstanding Class B Class B Common Stock Common Class B [Member] Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Property, plant and equipment, gross Property, Plant and Equipment, Gross Long-term Purchase Commitments [Table] Long-Term Purchase Commitment [Table] Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Increase (decrease) in contract liability (long-term) Increase (Decrease) In Contract With Customer, Liability, Noncurrent Increase (Decrease) In Contract With Customer, Liability, Noncurrent City Area Code City Area Code Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] All Trading Arrangements All Trading Arrangements [Member] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Schedule of Contract Balances Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] Investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Net cash provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Equity Method Investments and Joint Ventures [Abstract] Equity Method Investments and Joint Ventures [Abstract] Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] PEO Total Compensation Amount PEO Total Compensation Amount Money market funds Money Market Funds [Member] Equity Components [Axis] Equity Components [Axis] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Trade Accounts Receivable Trade Accounts Receivable [Member] Number of reportable segments Number of Reportable Segments Reclassification of pension and postretirement adjustments into earnings, net of $14 and $31 tax benefit in the three months ended March 31, 2026 and 2025, respectively. Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Contract asset (current) Change in Contract with Customer, Current Asset [Abstract] Change in Contract with Customer, Current Asset Other Performance Measure, Amount Other Performance Measure, Amount Entity Address, State or Province Entity Address, State or Province Related Party [Domain] Related and Nonrelated Parties [Domain] Total current liabilities Liabilities, Current Bellaire Bellaire Corporation [Member] Bellaire Corporation [Member] Earnings of unconsolidated operations Earnings of unconsolidated operations Earnings Of Unconsolidated Mines This item represents the entity's proportionate share for the period of the pre-tax earnings of the unconsolidated mines to which the equity method of accounting is applied. Income tax amounts are included in the income tax provision line. Individual: Individual [Axis] Other current assets Other Assets, Current Capital in excess of par value Additional Paid in Capital, Common Stock Entities [Table] Entities [Table] Effect of Change in Accounting Estimate, Type [Axis] Effect of Change in Accounting Estimate, Type [Axis] Entity [Domain] Entity [Domain] Net cash used for investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Expected timing of satisfaction Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Document Fiscal Year Focus Document Fiscal Year Focus Variable interest entity, reporting entity involvement, maximum risk of loss Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Operating lease right-of-use assets Operating Lease, Right-of-Use Asset Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Reductions to long-term debt Repayments of Long-Term Debt Entity Interactive Data Current Entity Interactive Data Current Reported Value Measurement Reported Value Measurement [Member] Equity [Abstract] Equity [Abstract] Business Segments Segment Reporting Disclosure [Text Block] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Current maturities of long-term debt Long-Term Debt and Lease Obligation, Current Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Amortization of intangible assets Amortization of Intangible Assets Selling, general and administrative expenses Selling, General and Administrative Expense Deferred income taxes Deferred Income Tax Assets, Net Exchange [Domain] Exchange [Domain] Depreciation, depletion and amortization Depreciation, depletion and amortization Depreciation, Depletion and Amortization Other non-current assets Other Assets, Noncurrent Customer [Domain] Customer [Domain] Entity Central Index Key Entity Central Index Key PEO Name PEO Name Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Change in Accounting Principle, Type [Axis] Change in Accounting Principle, Type [Axis] Closed mine obligations Nonoperating (Income) Expense, Closed Mine Obligations Nonoperating (Income) Expense, Closed Mine Obligations Arrangement Duration Trading Arrangement Duration Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Balance at January 1, 2026 Balance at March 31, 2026 Contract with Customer, Liability, Current Advance paid Long Term Contract Asset, Customer Advance Long Term Contract Asset, Customer Advance Segments [Axis] Segments [Axis] Change in Accounting Principle, Type [Domain] Change in Accounting Principle, Type [Domain] Exercise Price Award Exercise Price Coal Energy Related Inventory, Coal Entity Filer Category Entity Filer Category Local Phone Number Local Phone Number Class of Stock [Domain] Class of Stock [Domain] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Balance at January 1, 2026 Balance at March 31, 2026 Contract with Customer, Asset, after Allowance for Credit Loss, Noncurrent Other Payment for (Proceeds from) Other Investing Activity Increase (decrease) in contract liability (current) Increase (Decrease) In Contract With Customer, Liability, Current Increase (Decrease) In Contract With Customer, Liability, Current ASSETS Assets [Abstract] Mining supplies inventory Inventory, Noncurrent Nonrelated Party Nonrelated Party [Member] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Statement of Financial Position Location, Balance [Axis] Statement of Financial Position Location, Balance [Axis] Utility Coal Mining and Contract Mining Member Utility Coal Mining And Contract Mining Member [Member] Utility Coal Mining And Contract Mining Member Fair Value Disclosure Fair Value Disclosures [Text Block] Stockholders' equity Equity, Attributable to Parent [Abstract] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Entity Address, Address Line One Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Two  Total inventories Inventory, Current And Noncurrent Inventory, Current And Noncurrent Investment, Name [Axis] Investment, Name [Axis] Operating lease liabilities Operating Lease, Liability, Noncurrent Fair Value as of Grant Date Award Grant Date Fair Value Equity securities cost Equity Securities, FV-NI, Cost Entity Registrant Name Entity Registrant Name Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] NEW YORK STOCK EXCHANGE, INC. NEW YORK STOCK EXCHANGE, INC. [Member] Contract liability (current) Change in Contract with Customer, Current Liability [Roll Forward] Change in Contract with Customer, Current Liability [Roll Forward] Document Quarterly Report Document Quarterly Report Total assets accounted for at fair value on a recurring basis Assets, Fair Value Disclosure Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Number of operating segments Number of Operating Segments Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Net additions (reductions) to revolving credit agreement Line of Credit Facility, Increase (Decrease), Net Related Party [Axis] Related and Nonrelated Parties [Axis] Title Trading Arrangement, Individual Title Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Restatement Determination Date: Restatement Determination Date [Axis] Stock-based compensation Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Non-PEO NEO Non-PEO NEO [Member] Asset retirement obligations Asset Retirement Obligation, Current Reclassification adjustment to net income, net of tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Name Trading Arrangement, Individual Name All Award Types Award Type [Domain] Equity Awards Adjustments Equity Awards Adjustments [Member] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Compensation Amount Outstanding Recovery Compensation Amount Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Third-party lessee payment lag for oil and natural gas delivery Third-Party Lessee Payment Lag for Oil and Natural Gas Delivery Third-Party Lessee Payment Lag for Oil and Natural Gas Delivery Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Estimate of Fair Value Measurement Estimate of Fair Value Measurement [Member] Share Repurchase Program [Axis] Share Repurchase Program [Axis] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Name Awards Close in Time to MNPI Disclosures, Individual Name Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] 2023 Stock Repurchase Program 2023 Stock Repurchase Program [Member] 2023 Stock Repurchase Program Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Long-term Purchase Commitment [Line Items] Long-Term Purchase Commitment [Line Items] Segments [Domain] Segments [Domain] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Statement of Financial Position Location, Balance [Domain] Statement of Financial Position Location, Balance [Domain] Inventories Inventory, Net Eiger, LLC Eiger, LLC [Member] Eiger, LLC Company Selected Measure Name Company Selected Measure Name Fair Value Measurement [Domain] Fair Value Measurement [Domain] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Accounts payable and accounts payable to affiliates Accounts Payable, Current Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] All Executive Categories All Executive Categories [Member] Collateral Held [Domain] Collateral Held [Domain] Contract liability (long-term) Change in Contract with Customer, Noncurrent Liability [Roll Forward] Change in Contract with Customer, Noncurrent Liability [Roll Forward] Contingencies Commitments and Contingencies Disclosure [Text Block] Asset retirement obligations Mine Reclamation and Closing Liability, Noncurrent Diluted weighted average shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Diluted Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Reclassification of pension and postretirement adjustments into earnings, tax benefit Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax Entity Small Business Entity Small Business Operating expenses Operating expenses (income) Other Cost and Expense, Operating Document Transition Report Document Transition Report Ownership (percent) Ownership percent (less than) Equity Method Investment, Ownership Percentage Underlying Securities Award Underlying Securities Amount Equity Component [Domain] Equity Component [Domain] Document Period End Date Document Period End Date PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Long-term revolving credit agreements Long-Term Line of Credit, Noncurrent Shares Outstanding Class A Class A Common Stock Common Class A [Member] Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Document Type Document Type Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Name Outstanding Recovery, Individual Name Thacker Pass Thacker Pass [Member] Thacker Pass Revenues Total revenues Revenues Revenue from Contract with Customer, Excluding Assessed Tax All Individuals All Individuals [Member] Effect of Change in Accounting Estimate, Type [Domain] Effect of Change in Accounting Estimate, Type [Domain] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Transferred at a point in time Transferred at Point in Time [Member] Name Forgone Recovery, Individual Name Total current assets Assets, Current Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Stock repurchase program, authorized amount Share Repurchase Program, Authorized, Amount Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Change In Accounting Principle, Depreciation Method Change In Accounting Principle, Depreciation Method [Member] Change In Accounting Principle, Depreciation Method Purchase of treasury shares Purchase of treasury shares Treasury Stock, Value, Acquired, Par Value Method Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Entity Information [Line Items] Entity Information [Line Items] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Share Repurchase Program [Domain] Share Repurchase Program [Domain] Total other comprehensive income Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Contract asset (long-term) Change In Contract With Customer, Noncurrent Asset [Roll Forward] Change In Contract With Customer, Noncurrent Asset Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Production performance obligations satisfied in prior period, amount recognized Contract With Customer, Variable Consideration From Uncertainty Of Collectability, Production Performance Obligation Satisfied in Previous Period Contract With Customer, Variable Consideration From Uncertainty Of Collectability, Production Performance Obligation Satisfied in Previous Period Quoted Prices in Active Markets for Identical Assets (Level 1) Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Long-term debt Long-Term Debt And Lease Obligation, Excluding Long Term Line Of Credit Long-Term Debt And Lease Obligation, Excluding Long Term Line Of Credit Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Trade accounts receivable and accounts receivable from affiliates Accounts Receivable, after Allowance for Credit Loss, Current Title of 12(b) Security Title of 12(b) Security Prepaid insurance Prepaid Insurance Consolidation Items [Domain] Consolidation Items [Domain] Other (income) expense Nonoperating Income (Expense) Return of equity method investment Distributions received Proceeds from Equity Method Investment, Distribution, Return of Capital Minerals and Royalties Minerals And Royalties [Member] Minerals And Royalties Unconsolidated Subsidiaries Eiger Resources Equity Method Investments and Joint Ventures Disclosure [Text Block] Gross profit Gross Profit Equity securities Equity securities Equity Securities, FV-NI, Noncurrent Change in Accounting Method Accounted for as Change in Estimate Change in Accounting Method Accounted for as Change in Estimate [Member] Shares repurchased (in shares) Treasury Stock, Shares, Acquired Award Timing Disclosures [Line Items] Change in prior period estimate Contract With Customer, Variable Consideration From Uncertainty Of Collectability, Change In Prior Period Estimate Contract With Customer, Variable Consideration From Uncertainty Of Collectability, Change In Prior Period Estimate Schedule of Inventories Schedule of Inventory, Current [Table Text Block] Utility Coal Mining Utility Coal Mining [Member] Coal Mining [Member] Property, plant and equipment, net Property, Plant, and Equipment and Finance Lease Right-of-Use Asset, after Accumulated Depreciation and Amortization Assets held for sale Asset, Held-for-Sale, Not Part of Disposal Group Stockholders' Equity Equity [Text Block] Net income Net income Net income Net Income (Loss) Attributable to Parent Expiration Date Trading Arrangement Expiration Date Timing of Transfer of Good or Service [Axis] Timing of Transfer of Good or Service [Axis] Schedule of Equity Method Investment Equity Method Investments [Table Text Block] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Purchase of treasury shares Payments for Repurchase of Common Stock Adoption Date Trading Arrangement Adoption Date Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Entity Current Reporting Status Entity Current Reporting Status Eliminations Consolidation, Eliminations [Member] Customer [Axis] Customer [Axis] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Collateral Held [Axis] Collateral Held [Axis] Operating profit Operating profit (loss) Operating Income (Loss) Retained earnings Retained Earnings (Accumulated Deficit) Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Executive Category: Executive Category [Axis] Transferred over time Transferred over Time [Member] Current Fiscal Year End Date Current Fiscal Year End Date Class of Stock [Axis] Class of Stock [Axis] Prepaid profit sharing Prepaid Profit Sharing, Current Prepaid Profit Sharing, Current Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Other, net Other Nonoperating Income (Expense) Statement [Table] Statement [Table] Additions to note payable to affiliate Proceeds from (Repayments of) Notes Payable Financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Money market funds Equity Securities, FV-NI, Current Timing of Transfer of Good or Service [Domain] Timing of Transfer of Good or Service [Domain] Nature of Operations and Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Cash and cash equivalents Cash and Cash Equivalents [Abstract] All Adjustments to Compensation All Adjustments to Compensation [Member] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Amendment Flag Amendment Flag Termination Date Trading Arrangement Termination Date Net cash provided by (used for) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Measure: Measure [Axis] Unconsolidated Subsidiaries Unconsolidated Subsidiaries [Member] Unconsolidated Subsidiaries Receivable [Domain] Receivable [Domain] Basic weighted average shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Inventory impairment charge Inventory Write-down Cost of sales Cost of sales Cost of Product and Service Sold Segment Reporting [Abstract] Segment Reporting [Abstract] Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Other expense (income) Nonoperating Income (Expense) [Abstract] Entity Tax Identification Number Entity Tax Identification Number Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share (Gain) loss on equity securities Gain (loss) on equity securities Equity Securities, FV-NI, Realized Gain (Loss) Cash dividends paid Payments of Dividends Interest expense Interest Expense, Nonoperating Trading Arrangement: Trading Arrangement [Axis] Total Shareholder Return Amount Total Shareholder Return Amount Insider Trading Arrangements [Line Items] Unallocated Items Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] Security Exchange Name Security Exchange Name Equity securities Equity Securities [Member] Total liabilities Liabilities Common stock Common Stock, Value, Outstanding Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Operating expenses Operating Expenses [Abstract] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Other current liabilities Other Liabilities, Current Retirement benefit plans Liability, Defined Benefit Plan, Noncurrent Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Statement [Line Items] Statement [Line Items] Consolidation Items [Axis] Consolidation Items [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Cash and cash equivalents Cash and Cash Equivalent Common Stock Common Stock [Member] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Non-NEOs Non-NEOs [Member] Total increase (decrease) for the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation