Legal Settlement, Royalty Credit Realization Period

No definition available.

Name:	aci_LegalSettlementRoyaltyCreditRealizationPeriod
Namespace Prefix:	aci_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Legal Settlement, Royalty Credits

+ References

No definition available.

+ Details

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

- Definition

Legal Settlement, Royalty Credits, Percentage Of Royalty Expense

+ References

No definition available.

+ Details

Name:	aci_LegalSettlementRoyaltyCreditsPercentageOfRoyaltyExpense
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Payment to Acquire Investment, Closing Fees

+ References

No definition available.

+ Details

Name:	aci_PaymenttoAcquireInvestmentClosingFees
Namespace Prefix:	aci_
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_GainContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

+ Details

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

- Definition

The sum of amounts currently receivable other than from customers. 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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

The cash outflow associated with the purchase of all investments (debt, security, other) during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585

+ Details

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

- Details

Name:	us-gaap_GainContingenciesByNatureAxis=aci_PreferenceRightsLeaseApplicationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Preference Rights Lease Application Settlement Income (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Oct. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Gain Contingencies [Line Items]
Gain related to settlement				$ 39,000	$ 23,518	$ 39,000
Preference Rights Lease Application (PRLA)
Gain Contingencies [Line Items]
Gain related to settlement			$ 8,700	39,000	25,800
Royalty credits to be received due to settlement		$ 5,000		$ 67,000
Royalty credits to be received due to settlement, percentage of royalty obligations				50.00%
One-time payment for ownership interest	$ 27,000
Payment to acquire investment, closing fees	$ 1,000
Receivable			$ 27,800		$ 27,800
Minimum \| Preference Rights Lease Application (PRLA)
Gain Contingencies [Line Items]
Royalty credits to be received due to settlement, period of recognition		18 months
Maximum \| Preference Rights Lease Application (PRLA)
Gain Contingencies [Line Items]
Royalty credits to be received due to settlement, period of recognition		24 months