The value (monetary amount) of the amount the plaintiff was awarded in the legal matter.

No definition available.

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- Definition

Damages And Interest Awarded, Future Damages

+ References

No definition available.

+ Details

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- Definition

Damages And Interest Awarded, Past Damages

+ References

No definition available.

+ Details

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- Definition

Purchase Replacement Coal

+ References

No definition available.

+ Details

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- Definition

Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs.

+ References

+ Details

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- Definition

Carrying amount of reserve for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs, which is expected to be paid within one year of the date of the statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.25)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 25

-Article 5

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 55

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=6396582&loc=d3e15243-108350

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- Definition

Amount of judgment or settlement awarded to (against) the entity in respect of litigation.

+ References

No definition available.

+ Details

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- Definition

Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

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- Definition

The amount of cash paid for the settlement of litigation or for other legal issues during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

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Commitments and Contingencies (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended	1 Months Ended	0 Months Ended
	Jul. 31, 2013	Mar. 31, 2015	Feb. 01, 2011	Apr. 06, 2015	Feb. 16, 2015	Dec. 31, 2014	Aug. 25, 2011	May 02, 2011	Aug. 13, 2007
Purchase replacement coal, excess of									$ 100,000,000aci_PurchaseReplacementCoal
Damages and interest awarded							104,100,000aci_DamagesAndInterestAwarded	104,100,000aci_DamagesAndInterestAwarded
Damages and interest awarded, past damages							13,800,000
Damages and interest awarded, future damages					7,500,000		90,300,000
Payments for legal settlements	15,600,000us-gaap_PaymentsForLegalSettlements
Settlement accrual, provision		2,800,000us-gaap_LossContingencyAccrualProvision
Amount accrued		17,300,000us-gaap_LitigationReserve				22,300,000us-gaap_LitigationReserve
Amount accrued current portion		17,300,000us-gaap_LitigationReserveCurrent				10,100,000us-gaap_LitigationReserveCurrent
Minimum [Member]
Aggregate amount of past and future damages			228,000,000
Maximum [Member]
Aggregate amount of past and future damages			377,000,000
Subsidiaries [Member] \| Subsequent Event [Member]
Litigation settlement, amount				$ (15,000,000)