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

No definition available.

+ Details

Name:	aci_InsuranceRecoveryHydraulicShieldsNotRecoverable
Namespace Prefix:	aci_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	aci_InsuranceRecoverySystemsNotRecoverable
Namespace Prefix:	aci_
Data Type:	xbrli:integerItemType
Balance Type:	na
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_BusinessInterruptionLossLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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 http://asc.fasb.org/extlink&oid=114868897&loc=SL114872001-224240

+ Details

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

- Details

Name:	us-gaap_BusinessInterruptionLossAxis=aci_MountainLaurelLongwallOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Gain on Property Insurance Recovery Related to Mountain Laurel Longwall (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2020 item
Business Interruption Loss [Line Items]
Gain on Business Interruption Insurance Recovery \| $		$ 23,518	$ 0
Longwall system's not recoverable \| item				123
Mountain Laurel Longwall Operations
Business Interruption Loss [Line Items]
Gain on Business Interruption Insurance Recovery \| $	$ 23,500
Hydraulic Shields not recoverable \| item				176