Defined Benefit Plan, Expected Future Employer Contributions

No definition available.

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

- Definition

Multiemployer Plan, Pension, Additional Funding

+ References

No definition available.

+ Details

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

- Definition

Multiemployer Plans, Plan Benefits, Financial Assistance Received From Pension Benefit Guaranty Corporation

+ References

No definition available.

+ Details

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

- Definition

Share-based Compensation Arrangement By Share-based Payment Award, Aggregate Retention Program Amount

+ References

No definition available.

+ Details

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

- Definition

Share-Based Compensation Arrangement by Share-Based Payment Award, Business Combination, Award Vesting Rights, Percentage

+ References

No definition available.

+ Details

Name:	aci_ShareBasedCompensationArrangementByShareBasedPaymentAwardBusinessCombinationAwardVestingRightsPercentage
Namespace Prefix:	aci_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-Based Compensation Arrangement by Share-Based Payment Award, Business Combination Termination, Award Vesting Rights, Percentage

+ References

No definition available.

+ Details

Name:	aci_ShareBasedCompensationArrangementByShareBasedPaymentAwardBusinessCombinationTerminationAwardVestingRightsPercentage
Namespace Prefix:	aci_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of (increase) decrease in accumulated benefit obligation of defined benefit plan from effects of settlement and curtailment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (r)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

+ Details

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

- Definition

Amount of increase (decrease) in plan assets of defined benefit plan from assets transferred into (from) plan.

+ References

No definition available.

+ Details

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

- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI https://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI https://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

+ Details

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

- Definition

Amount of contribution expected to be received by defined benefit plan from employer in remainder of current fiscal year. Excludes contribution previously paid by employer in current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI https://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921

+ Details

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

- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(7)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3A
-URI https://asc.fasb.org/extlink&oid=123453770&loc=SL108413299-114919

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)(7)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
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_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=aci_MergerAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

EMPLOYEE BENEFIT PLANS - Narrative (Details) - USD ($) $ in Millions			3 Months Ended		4 Months Ended	9 Months Ended
	Dec. 18, 2022	Aug. 08, 2022	Dec. 03, 2022	Dec. 04, 2021	Jun. 18, 2022	Dec. 03, 2022	Dec. 04, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution made to defined benefit plan			$ 14.1	$ 16.8		$ 19.1	$ 28.0
Expected future employer contributions for remainder of the fiscal year			$ 7.9			7.9
Defined benefit plan, transferred						203.5
Defined benefit pension obligation						205.4
Defined benefit plan gain due to settlement							$ 11.1
PBGC's financial assistance					$ 1,200.0
Defined contribution, required annual contribution		$ 120.0
Merger Agreement \| Subsequent Event
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Aggregate retention program	$ 100.0
Merger Agreement \| Subsequent Event \| Share-Based Payment Arrangement, Tranche One
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Business combination, award vesting rights, percentage	50.00%
Business combination termination, award vesting rights	50.00%
Merger Agreement \| Subsequent Event \| Share-Based Payment Arrangement, Tranche Two
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Business combination, award vesting rights, percentage	50.00%
Business combination termination, award vesting rights	50.00%
Other post-retirement benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Non-cash pre-tax charge						$ 19.0