Name:	arch_InsuranceRecoveryHydraulicShieldsNotRecoverable
Namespace Prefix:	arch_
Data Type:	xbrli:integerItemType
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Period Type:	instant

Represents longwall system's not recovered as a result of geologic conditions in the final longwall panel.

No definition available.

Name:	arch_InsuranceRecoverySystemsNotRecoverable
Namespace Prefix:	arch_
Data Type:	xbrli:integerItemType
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Period Type:	instant

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_BusinessInterruptionLossLineItems
Namespace Prefix:	us-gaap_
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Period Type:	duration

The net amount (to the extent disclosed within this portion of the income statement) by which an insurance settlement exceeds incremental costs incurred from the event causing an interruption of business, plus the insurance award for earnings lost from the event, such as a natural catastrophe, explosion or fire.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=114868897&loc=SL114872001-224240

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

The amount recovered from insurance. These recoveries reduce costs and losses that are reported as a separate line item under operating expenses.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227