2.4.1.9 Html 164 251 1 false 59 0 false 7 false false R1.htm 0001000 - Document - Document And Entity Information Sheet http://archcoal.com/role/DocumentAndEntityInformation Document And Entity Information false false R2.htm 1001000 - Statement - Condensed Consolidated Statements of Operations Sheet http://archcoal.com/role/CondensedConsolidatedStatementsOfOperations Condensed Consolidated Statements of Operations false false R3.htm 1002000 - Statement - Condensed Consolidated Statements of Comprehensive Income (Loss) Sheet http://archcoal.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeLoss Condensed Consolidated Statements of Comprehensive Income (Loss) true false R4.htm 1003000 - Statement - Condensed Consolidated Balance Sheets Sheet http://archcoal.com/role/CondensedConsolidatedBalanceSheets Condensed Consolidated Balance Sheets false false R5.htm 1003501 - Statement - Condensed Consolidated Balance Sheets (Parenthetical) Sheet http://archcoal.com/role/CondensedConsolidatedBalanceSheetsParenthetical Condensed Consolidated Balance Sheets (Parenthetical) false false R6.htm 1007000 - Statement - Condensed Consolidated Statements of Cash Flows Sheet http://archcoal.com/role/CondensedConsolidatedStatementsOfCashFlows Condensed Consolidated Statements of Cash Flows false false R7.htm 2101100 - Disclosure - Basis of Presentation Sheet http://archcoal.com/role/BasisOfPresentation Basis of Presentation false false R8.htm 2104100 - Disclosure - Accounting Policies Sheet http://archcoal.com/role/AccountingPolicies Accounting Policies false false R9.htm 2106100 - Disclosure - Accumulated Other Comprehensive Income Sheet http://archcoal.com/role/AccumulatedOtherComprehensiveIncome Accumulated Other Comprehensive Income false false R10.htm 2108100 - Disclosure - Divestitures Sheet http://archcoal.com/role/Divestitures Divestitures false false R11.htm 2109100 - Disclosure - Inventories Sheet http://archcoal.com/role/Inventories Inventories false false R12.htm 2111100 - Disclosure - Investments in Available-for-Sale Securities Sheet http://archcoal.com/role/InvestmentsInAvailableForSaleSecurities Investments in Available-for-Sale Securities false false R13.htm 2112100 - Disclosure - Derivatives Sheet http://archcoal.com/role/Derivatives Derivatives false false R14.htm 2113100 - Disclosure - Accrued Expenses and Other Current Liabilities Sheet http://archcoal.com/role/AccruedExpensesAndOtherCurrentLiabilities Accrued Expenses and Other Current Liabilities false false R15.htm 2114100 - Disclosure - Debt and Financing Arrangements Sheet http://archcoal.com/role/DebtAndFinancingArrangements Debt and Financing Arrangements false false R16.htm 2118100 - Disclosure - Income Taxes Sheet http://archcoal.com/role/IncomeTaxes Income Taxes false false R17.htm 2120100 - Disclosure - Fair Value Measurements Sheet http://archcoal.com/role/FairValueMeasurements Fair Value Measurements false false R18.htm 2121100 - Disclosure - Loss Per Common Share Sheet http://archcoal.com/role/LossPerCommonShare Loss Per Common Share false false R19.htm 2123100 - Disclosure - Employee Benefit Plans Sheet http://archcoal.com/role/EmployeeBenefitPlans Employee Benefit Plans false false R20.htm 2125100 - Disclosure - Commitments and Contingencies Sheet http://archcoal.com/role/CommitmentsAndContingencies Commitments and Contingencies false false R21.htm 2130100 - Disclosure - Segment Information Sheet http://archcoal.com/role/SegmentInformation Segment Information false false R22.htm 2131100 - Disclosure - Supplemental Consolidating Financial Information Sheet http://archcoal.com/role/SupplementalConsolidatingFinancialInformation Supplemental Consolidating Financial Information false false R23.htm 2201201 - Disclosure - Basis of Presentation (Policy) Sheet http://archcoal.com/role/BasisOfPresentationPolicy Basis of Presentation (Policy) false false R24.htm 2306301 - Disclosure - Accumulated Other Comprehensive Income (Tables) Sheet http://archcoal.com/role/AccumulatedOtherComprehensiveIncomeTables Accumulated Other Comprehensive Income (Tables) false false R25.htm 2309301 - Disclosure - Inventories (Tables) Sheet http://archcoal.com/role/InventoriesTables Inventories (Tables) false false R26.htm 2311301 - Disclosure - Investments in Available-for-Sale Securities (Tables) Sheet http://archcoal.com/role/InvestmentsInAvailableForSaleSecuritiesTables Investments in Available-for-Sale Securities (Tables) false false R27.htm 2312301 - Disclosure - Derivatives (Tables) Sheet http://archcoal.com/role/DerivativesTables Derivatives (Tables) false false R28.htm 2313301 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables) Sheet http://archcoal.com/role/AccruedExpensesAndOtherCurrentLiabilitiesTables Accrued Expenses and Other Current Liabilities (Tables) false false R29.htm 2314301 - Disclosure - Debt and Financing Arrangements (Tables) Sheet http://archcoal.com/role/DebtAndFinancingArrangementsTables Debt and Financing Arrangements (Tables) false false R30.htm 2320301 - Disclosure - Fair Value Measurements (Tables) Sheet http://archcoal.com/role/FairValueMeasurementsTables Fair Value Measurements (Tables) false false R31.htm 2323301 - Disclosure - Employee Benefit Plans (Tables) Sheet http://archcoal.com/role/EmployeeBenefitPlansTables Employee Benefit Plans (Tables) false false R32.htm 2330301 - Disclosure - Segment Information (Tables) Sheet http://archcoal.com/role/SegmentInformationTables Segment Information (Tables) false false R33.htm 2331301 - Disclosure - Supplemental Consolidating Financial Information (Tables) Sheet http://archcoal.com/role/SupplementalConsolidatingFinancialInformationTables Supplemental Consolidating Financial Information (Tables) false false R34.htm 2406402 - Disclosure - Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) Sheet http://archcoal.com/role/AccumulatedOtherComprehensiveIncomeScheduleOfAccumulatedOtherComprehensiveIncomeLossDetails Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) false false R35.htm 2406403 - Disclosure - Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) Sheet http://archcoal.com/role/AccumulatedOtherComprehensiveIncomeScheduleOfReclassificationsDetails Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) false false R36.htm 2408401 - Disclosure - Divestitures (Narrative) (Details) Sheet http://archcoal.com/role/DivestituresNarrativeDetails Divestitures (Narrative) (Details) false false R37.htm 2409402 - Disclosure - Inventories (Details) Sheet http://archcoal.com/role/InventoriesDetails Inventories (Details) false false R38.htm 2411402 - Disclosure - Investments in Available-for-Sale Securities (Details) Sheet http://archcoal.com/role/InvestmentsInAvailableForSaleSecuritiesDetails Investments in Available-for-Sale Securities (Details) false false R39.htm 2411403 - Disclosure - Investments in Available-for-Sale Securities (Narrative) (Details) Sheet http://archcoal.com/role/InvestmentsInAvailableForSaleSecuritiesNarrativeDetails Investments in Available-for-Sale Securities (Narrative) (Details) false false R40.htm 2412402 - Disclosure - Derivatives (Narrative) (Details) Sheet http://archcoal.com/role/DerivativesNarrativeDetails Derivatives (Narrative) (Details) false false R41.htm 2412403 - Disclosure - Derivatives (Schedule of Price Risk Derivatives) (Details) Sheet http://archcoal.com/role/DerivativesScheduleOfPriceRiskDerivativesDetails Derivatives (Schedule of Price Risk Derivatives) (Details) false false R42.htm 2412404 - Disclosure - Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) Sheet http://archcoal.com/role/DerivativesDisclosureOfFairValueOfDerivativesDetails Derivatives (Disclosure Of Fair Value Of Derivatives) (Details) false false R43.htm 2412405 - Disclosure - Derivatives (Net Derivatives As Reflected On The Balance Sheets) (Details) Sheet http://archcoal.com/role/DerivativesNetDerivativesAsReflectedOnBalanceSheetsDetails Derivatives (Net Derivatives As Reflected On The Balance Sheets) (Details) false false R44.htm 2412406 - Disclosure - Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) Sheet http://archcoal.com/role/DerivativesEffectsOfDerivativesOnMeasuresOfFinancialPerformanceDetails Derivatives (Effects Of Derivatives On Measures Of Financial Performance) (Details) false false R45.htm 2413402 - Disclosure - Accrued Expenses and Other Current Liabilities (Details) Sheet http://archcoal.com/role/AccruedExpensesAndOtherCurrentLiabilitiesDetails Accrued Expenses and Other Current Liabilities (Details) false false R46.htm 2414402 - Disclosure - Debt and Financing Arrangements (Details) Sheet http://archcoal.com/role/DebtAndFinancingArrangementsDetails Debt and Financing Arrangements (Details) false false R47.htm 2418401 - Disclosure - Income Taxes (Narrative) (Details) Sheet http://archcoal.com/role/IncomeTaxesNarrativeDetails Income Taxes (Narrative) (Details) false false R48.htm 2420402 - Disclosure - Fair Value Measurements (Narrative) (Details) Sheet http://archcoal.com/role/FairValueMeasurementsNarrativeDetails Fair Value Measurements (Narrative) (Details) false false R49.htm 2420403 - Disclosure - Fair Value Measurements (Summary Of Financial Assets And Liabilities Accounted For At Fair Value) (Details) Sheet http://archcoal.com/role/FairValueMeasurementsSummaryOfFinancialAssetsAndLiabilitiesAccountedForAtFairValueDetails Fair Value Measurements (Summary Of Financial Assets And Liabilities Accounted For At Fair Value) (Details) false false R50.htm 2420404 - Disclosure - Fair Value Measurements (Summary Of Change In The Fair Values Of Financial Instruments Categorized As Level 3) (Details) Sheet http://archcoal.com/role/FairValueMeasurementsSummaryOfChangeInFairValuesOfFinancialInstrumentsCategorizedAsLevel3Details Fair Value Measurements (Summary Of Change In The Fair Values Of Financial Instruments Categorized As Level 3) (Details) false false R51.htm 2421401 - Disclosure - Loss Per Common Share (Narrative) (Details) Sheet http://archcoal.com/role/LossPerCommonShareNarrativeDetails Loss Per Common Share (Narrative) (Details) false false R52.htm 2423402 - Disclosure - Employee Benefit Plans (Pension And Other Postretirement Benefit Costs) (Details) Sheet http://archcoal.com/role/EmployeeBenefitPlansPensionAndOtherPostretirementBenefitCostsDetails Employee Benefit Plans (Pension And Other Postretirement Benefit Costs) (Details) false false R53.htm 2425401 - Disclosure - Commitments and Contingencies (Narrative) (Details) Sheet http://archcoal.com/role/CommitmentsAndContingenciesNarrativeDetails Commitments and Contingencies (Narrative) (Details) false false R54.htm 2430402 - Disclosure - Segment Information (Schedule Of Operating Segment Results) (Details) Sheet http://archcoal.com/role/SegmentInformationScheduleOfOperatingSegmentResultsDetails Segment Information (Schedule Of Operating Segment Results) (Details) false false R55.htm 2430403 - Disclosure - Segment Information (Reconciliation Statement Of Segment Income from Operations To Consolidated Income Before Income Taxes) (Details) Sheet http://archcoal.com/role/SegmentInformationReconciliationStatementOfSegmentIncomeFromOperationsToConsolidatedIncomeBeforeIncomeTaxesDetails Segment Information (Reconciliation Statement Of Segment Income from Operations To Consolidated Income Before Income Taxes) (Details) false false R56.htm 2431402 - Disclosure - Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Operations) (Details) Sheet http://archcoal.com/role/SupplementalConsolidatingFinancialInformationScheduleOfCondensedConsolidatingStatementsOfOperationsDetails Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Operations) (Details) false false R57.htm 2431403 - Disclosure - Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) Sheet http://archcoal.com/role/SupplementalConsolidatingFinancialInformationScheduleOfCondensedConsolidatingBalanceSheetsDetails Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) false false R58.htm 2431404 - Disclosure - Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Cash Flows) (Details) Sheet http://archcoal.com/role/SupplementalConsolidatingFinancialInformationScheduleOfCondensedConsolidatingStatementsOfCashFlowsDetails Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Cash Flows) (Details) false false All Reports Book All Reports 'Monetary' elements on report '2409402 - Disclosure - Inventories (Details)' had a mix of different decimal attribute values. 'Monetary' elements on report '2414402 - Disclosure - Debt and Financing Arrangements (Details)' had a mix of different decimal attribute values. 'Monetary' elements on report '2420402 - Disclosure - Fair Value Measurements (Narrative) (Details)' had a mix of different decimal attribute values. Process Flow-Through: 1001000 - Statement - Condensed Consolidated Statements of Operations Process Flow-Through: 1002000 - Statement - Condensed Consolidated Statements of Comprehensive Income (Loss) Process Flow-Through: 1003000 - Statement - Condensed Consolidated Balance Sheets Process Flow-Through: Removing column 'Mar. 31, 2014' Process Flow-Through: Removing column 'Dec. 31, 2013' Process Flow-Through: 1003501 - Statement - Condensed Consolidated Balance Sheets (Parenthetical) Process Flow-Through: 1007000 - Statement - Condensed Consolidated Statements of Cash Flows aci-20150331.xml aci-20150331.xsd aci-20150331_cal.xml aci-20150331_def.xml aci-20150331_lab.xml aci-20150331_pre.xml true true