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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Percentage of coal operators need to post security which is equal to projected back lung liabilities.

No definition available.

Name:	arch_CoalOperatorsMustPostSecurityPercentage
Namespace Prefix:	arch_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Increase (decrease) in workers compensation obligations due to change in discount rate.

No definition available.

Name:	arch_IncreaseDecreaseWorkersCompensationObligationsChangeInDiscountRate
Namespace Prefix:	arch_
Data Type:	xbrli:monetaryItemType
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Percentage of self-insured operators need to post security which is equal to projected back lung liabilities.

No definition available.

Name:	arch_SelfInsuredOperatorsMustPostSecurityPercentage
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Surety bonds and letters of credit outstanding for future obligations.

No definition available.

Name:	arch_SuretyBondsAndLettersOfCreditOutstanding
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Data Type:	xbrli:monetaryItemType
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Represents the number of days required to submit additional collateral to retain self-insurance status.

No definition available.

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Value of input used to measure debt instrument, including, but not limited to, convertible and non-convertible debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
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Data Type:	xbrli:decimalItemType
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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.

No definition available.

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
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Data Type:	xbrli:stringItemType
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Period Type:	duration

Amount due in settlement of a claim for reimbursement from an insurance company when the Company has suffered a loss covered under an insurance policy. For classified balance sheets, represents the current amount receivable, that is amounts expected to be collected within one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(a)(4))
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Data Type:	xbrli:monetaryItemType
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Carrying amount due after one year of the balance sheet date (or one operating cycle, if longer) of amount due in settlement of a claim for reimbursement from an insurance company when the Company has suffered a loss covered under an insurance policy.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of accrued obligation to policyholders that relates to insured events under disability and accident insurance contracts.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 25
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-URI https://asc.fasb.org/extlink&oid=124504495&loc=d3e13145-158430

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-Subparagraph (SX 210.7-03.13(a)(1))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910