Equity method investment, percent of ownership interest acquired.

No definition available.

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

- Definition

Equity method investment, percent of ownership interest sold.

+ References

No definition available.

+ Details

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

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of the entity's equity method investment which has been sold.

+ References

No definition available.

+ Details

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

- Definition

The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ 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
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585

+ 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

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Period Type:	duration

- Details

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- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

- Details

Name:	srt_OwnershipAxis=aci_KnightHawkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=aci_DominionTerminalAssociatesMember
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Equity Method Investments and Membership Interests in Joint Ventures (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Payments to auction house to increase ownership			$ 1,700
Knight Hawk \| Sale
Schedule of Equity Method Investments [Line Items]
Ownership interest sold		1.00%
Ownership interest sold, value		$ 2,000
Knight Hawk
Schedule of Equity Method Investments [Line Items]
Ownership interest acquired	1.50%
Ownership interest	49.50%		49.50%
DTA
Schedule of Equity Method Investments [Line Items]
Ownership interest	35.00%		35.00%