Loss Contingency, Additional Claims, Number

No definition available.

Name:	aci_LossContingencyAdditionalClaimsNumber
Namespace Prefix:	aci_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount awarded to other party in judgment or settlement of litigation.

+ References

No definition available.

+ Details

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

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

- Definition

Number of claims settled.

+ References

+ Details

Name:	us-gaap_LossContingencyClaimsSettledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=121557415&loc=d3e14435-108349

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=121557415&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total number of new claims filed pertaining to a loss contingency during the period.

+ References

+ Details

Name:	us-gaap_LossContingencyNewClaimsFiledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of pending claims pertaining to a loss contingency.

+ References

+ Details

Name:	us-gaap_LossContingencyPendingClaimsNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_LitigationCaseAxis=aci_QuiTamLawsuitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_PendingLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_LitigationCaseAxis=aci_ConsolidatedCasesForMultidistrictLitigationMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_ThreatenedLitigationMember
Namespace Prefix:
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Period Type:

- Details

Name:	srt_CounterpartyNameAxis=aci_BlackfeetTribeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_SettledLitigationMember
Namespace Prefix:
Data Type:	na
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COMMITMENTS AND CONTINGENCIES AND OFF BALANCE SHEET ARRANGEMENTS - Legal Contingencies (Details) $ in Millions		3 Months Ended	12 Months Ended
	Feb. 17, 2023 USD ($)	Feb. 25, 2023 USD ($) case lawsuit	Feb. 25, 2023 USD ($) case lawsuit
Qui Tam Lawsuits \| Pending litigation
Loss Contingencies [Line Items]
Number of lawsuits filed against the company \| lawsuit		2	2
Qui Tam Lawsuits \| Pending litigation \| Minimum
Loss Contingencies [Line Items]
Damages sought in a lawsuit \| $			$ 100.0
Consolidated cases for multidistrict litigation
Loss Contingencies [Line Items]
Number of lawsuits filed against the company		2,000	2,000
Damages sought in a lawsuit \| $		$ 21.5
Additional claims		14
Claims settled		15
Consolidated cases for multidistrict litigation \| Threatened litigation \| Blackfeet Tribe
Loss Contingencies [Line Items]
Number of new claims filed			100
Consolidated cases for multidistrict litigation \| Settled Litigation
Loss Contingencies [Line Items]
Litigation claim settlement \| $	$ 107.0