Derivatives (Narrative) (Details) gal in Millions, $ in Millions	9 Months Ended
Sep. 30, 2021 USD ($) gal	Dec. 31, 2020 USD ($)
Derivative [Line Items]
Current asset representing cash collateral posted	$ 25.1	$ 1.4
Coal Contract
Derivative [Line Items]
Value of trading portfolio realized - gains (losses)	0.1
Call Option \| Heating Oil
Derivative [Line Items]
Notional Amount	$ 0.0
Not Designated as Hedging Instrument \| Diesel Purchases \| Minimum
Derivative [Line Items]
Diesel fuel purchased annually (in gallons) \| gal	35
Not Designated as Hedging Instrument \| Diesel Purchases \| Maximum
Derivative [Line Items]
Diesel fuel purchased annually (in gallons) \| gal	40

Gallons of diesel fuel purchased annually.

No definition available.

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Namespace Prefix:	aci_
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Balance Type:	na
Period Type:	duration

- Definition

Value of trading portfolio realized at a certain point in time gains (losses)

+ References

No definition available.

+ Details

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

- Definition

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41678-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

+ Details

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Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
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- Details

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- Details

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Data Type:	na
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- Details

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Data Type:	na
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- Details

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- Details

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