Legal Settlement, Royalty Credits

No definition available.

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

Royalty Credits, Utilized

+ References

No definition available.

+ Details

Name:	aci_RoyaltyCreditsUtilized
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_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=118942415&loc=d3e14326-108349

+ Details

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

- 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=118932676&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:

Preference Rights Lease Application (PRLA) Settlement Income (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gain Contingencies [Line Items]
Gain related to settlement		$ 39,000	$ 0	$ 0
Company utilization of royalty credits		13,300
Preference Rights Lease Application
Gain Contingencies [Line Items]
Gain related to settlement	$ 39,000
Royalty credits to be received due to settlement		$ 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