Debt Instrument, Amortization Payments, Quarterly Amount

No definition available.

Name:	aci_DebtInstrumentAmortizationPaymentsQuarterlyAmount
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt Instrument, Call Premium Rate

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentCallPremiumRate
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Covenants, Default to Indebtedness, Minimum

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentCovenantsDefaulttoIndebtednessMinimum
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt Instrument, Covenants, Default to Surety, Reclamation or Similar Bond, Minimum

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentCovenantsDefaulttoSuretyReclamationorSimilarBondMinimum
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt Instrument, Covenants, Default to Uninsured Judgment, Minimum

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentCovenantsDefaulttoUninsuredJudgmentMinimum
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt Instrument, Covenants, Pre-Payment, Cash Proceeds of Debt and Other Non-Ordinary Sales or Dispositions, Percent

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentCovenantsPrePaymentCashProceedsofDebtandOtherNonOrdinarySalesorDispositionsPercent
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Covenants, Uninsured Losses or Proceedings Against, Minimum

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentCovenantsUninsuredLossesorProceedingsAgainstMinimum
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt Instrument, Guarantees, Equity Interest of Domestic Subsidiaries, Percent

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentGuaranteesEquityInterestofDomesticSubsidiariesPercent
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Guarantees, Equity Interest of Foreign Subsidiaries, Percent

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentGuaranteesEquityInterestofForeignSubsidiariesPercent
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Issuance of Face Amount, Percentage

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentIssuanceofFaceAmountPercentage
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Term, Period Following Liquidity Threshold Met

+ References

No definition available.

+ Details

Name:	aci_DebtInstrumentTermPeriodFollowingLiquidityThresholdMet
Namespace Prefix:	aci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Letters of Credit, Additional Borrowing Capacity

+ References

No definition available.

+ Details

Name:	aci_LettersofCreditAdditionalBorrowingCapacity
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Covenant Compliance, Liquidity Amount

+ References

No definition available.

+ Details

Name:	aci_LineOfCreditFacilityCovenantComplianceLiquidityAmount
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Expiration Period, Period Following Maturity Of Secondary Credit Facility

+ References

No definition available.

+ Details

Name:	aci_LineOfCreditFacilityExpirationPeriodPeriodFollowingMaturityOfSecondaryCreditFacility
Namespace Prefix:	aci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line Of Credit Facility, Liquidity, Threshold Consecutive Days

+ References

No definition available.

+ Details

Name:	aci_LineOfCreditFacilityLiquidityThresholdConsecutiveDays
Namespace Prefix:	aci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Covenant Compliance, Borrowing Base, Percentage of Clause

+ References

No definition available.

+ Details

Name:	aci_LineofCreditFacilityCovenantComplianceBorrowingBasePercentageofClause
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Covenant Compliance, Borrowing Base, Percentage of Coal Inventory

+ References

No definition available.

+ Details

Name:	aci_LineofCreditFacilityCovenantComplianceBorrowingBasePercentageofCoalInventory
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Covenant Compliance, Borrowing Base, Percentage of Parts and Supplies Inventory

+ References

No definition available.

+ Details

Name:	aci_LineofCreditFacilityCovenantComplianceBorrowingBasePercentageofPartsandSuppliesInventory
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Increase (Decrease)

+ References

No definition available.

+ Details

Name:	aci_LineofCreditFacilityMaximumBorrowingCapacityIncreaseDecrease
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line of Credit, Liquidity Covenant, Amount

+ References

No definition available.

+ Details

Name:	aci_LineofCreditLiquidityCovenantAmount
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percent Of Eligible Cash

+ References

No definition available.

+ Details

Name:	aci_PercentOfEligibleCash
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
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-Name Accounting Standards Codification
-Topic 860
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-Section 50
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-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of net gains or losses on interest rate cash flow hedges reclassified during the period to earnings from accumulated other comprehensive income upon the hedged transaction affecting earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

+ Details

Name:	us-gaap_InterestRateCashFlowHedgeGainLossReclassifiedToEarningsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow for loan and debt issuance costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_PredecessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aci_TermLoanDebtFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=aci_TermLoanDebtFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=aci_RegionsBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=aci_ArchReceivableCompanyLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aci_AccountsReceivableSecuritizationFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
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Debt and Financing Arrangements (Company Debt Narrative) (Details) - USD ($)							3 Months Ended		9 Months Ended	12 Months Ended
	Nov. 19, 2018	Apr. 27, 2018	Apr. 03, 2018	Sep. 25, 2017	Apr. 27, 2017	Mar. 07, 2017	Sep. 30, 2017	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Letters of credit outstanding										$ 0
Additional availability for borrowings										11,200,000
Derivative assets										18,879,000	$ 55,550,000
Debt financing costs								$ 0		1,257,000	10,149,000
Predecessor
Debt Instrument [Line Items]
Debt financing costs									$ 23,011,000
Senior notes \| Term loan debt facility
Debt Instrument [Line Items]
Amount of debt instrument				$ 300,000,000
Call premium				1.00%
Mandatory prepayment amount						100.00%
Default to indebtedness amount						$ 50,000,000
Default to surety, reclamation, or similar bonds securing obligations						50,000,000
Default to uninsured judgements						50,000,000
Uninsured losses or proceedings against minimum amount						50,000,000
Senior notes \| Term loan debt facility \| LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate				3.25%
Line of credit \| Secured debt
Debt Instrument [Line Items]
Letters of credit outstanding										74,800,000
Term loan debt facility \| Senior notes
Debt Instrument [Line Items]
Amount of debt instrument						$ 300,000,000
Percent of face amount of debt issued						99.50%
Quarterly principal amortization payments						$ 750,000
Voting interests of directly owned subsidiaries						100.00%
Voting equity interests of directly owned foreign subsidiaries						65.00%
Term loan debt facility \| Senior notes \| LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate							2.75%
Term loan debt facility \| Senior notes \| LIBOR \| Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate				1.00%
Term loan debt facility \| Senior notes \| Base rate
Debt Instrument [Line Items]
Applicable margin on interest rate				2.25%
Term loan debt facility \| Line of credit
Debt Instrument [Line Items]
Call premium			1.00%
Term loan debt facility \| Line of credit \| LIBOR
Debt Instrument [Line Items]
Applicable margin on interest rate			2.75%
Term loan debt facility \| Line of credit \| LIBOR \| Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate			1.00%
Term loan debt facility \| Line of credit \| Base rate
Debt Instrument [Line Items]
Applicable margin on interest rate			1.75%
Regions Bank \| Line of credit \| Secured debt
Debt Instrument [Line Items]
Amount of debt instrument					$ 40,000,000
Maximum borrowing capacity	$ 50,000,000									200,000,000
Current borrowing capacity					160,000,000
Liquidity amount minimum					$ 175,000,000
Letters of credit outstanding										35,700,000
Borrowing base percentage, coal inventory					85.00%
Borrowing base percentage, parts and supplies inventory					85.00%
Borrowing base, percentage of clause					35.00%
Percent of eligible cash					100.00%
Increase to line of credit facility	$ 10,000,000
Covenant amount					$ 250,000,000
Regions Bank \| Line of credit \| Secured debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate					2.00%
Regions Bank \| Line of credit \| Secured debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Borrowing base, percentage of clause					2.50%
Regions Bank \| Line of credit \| Secured debt \| Base rate \| Minimum
Debt Instrument [Line Items]
Applicable margin on interest rate					1.00%
Regions Bank \| Line of credit \| Secured debt \| Base rate \| Maximum
Debt Instrument [Line Items]
Applicable margin on interest rate					1.50%
Arch Receivable Company, LLC \| Line of credit \| Secured debt \| Accounts Receivable Securitization Facility
Debt Instrument [Line Items]
Current borrowing capacity										53,500,000
Maturity date		3 years
Liquidity threshold, consecutive days		60 days
Liquidity threshold		364 days
Liquidity threshold, expiration period		60 days
Interest rate swap
Debt Instrument [Line Items]
Gain (loss) on interest rate swap										1,200,000	$ (100,000)
Interest rate swap \| Other noncurrent assets
Debt Instrument [Line Items]
Derivative assets										$ 2,500,000