Risk Concentrations (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2013 T	Dec. 31, 2012	Dec. 31, 2011
Concentration Risk [Line Items]
Sales to customers in foreign countries	$ 800,000	$ 1,153,100	$ 900,000
Receivables	229,573	332,080
Trade accounts receivable	198,020	247,539
Tons of coal sold	139,600,000
Percentage of tons sold under long term contract	59.00%
Long term contracts percentage of company revenue	49.00%
Long-term contracts, life term, minimum	1 year
Long-term contracts, life term, maximum	7 years
Electric Utilities [Member]
Concentration Risk [Line Items]
Receivables	125,700	159,500
Percentage of total trade accounts receivable	64.00%	65.00%
Domestic And Foreign Steel Producers [Member]
Concentration Risk [Line Items]
Trade accounts receivable	$ 70,500	$ 86,600
Trade receivables, percentage	36.00%	35.00%

Accounts receivables percentage of total trade

No definition available.

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

- Definition

Goods sold during the year

+ References

No definition available.

+ Details

Name:	aci_Goodssoldduringyear
Namespace Prefix:	aci_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum life term of a long-term sales contract.

+ References

No definition available.

+ Details

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

- Definition

The minimum life term of a long-term sales contract.

+ References

No definition available.

+ Details

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

- Definition

Percentage of total company revenue

+ References

No definition available.

+ Details

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

- Definition

Percentage of accounts receivable net current.

+ References

No definition available.

+ Details

Name:	aci_Percentageofaccountsreceivablenetcurrent
Namespace Prefix:	aci_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of goods sold under long term contract

+ References

No definition available.

+ Details

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

- Definition

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.3-4)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 3

-Subparagraph a(1)

-Article 5

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 4

-Article 5

+ Details

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

- Details

Name:	us-gaap_ConcentrationRiskLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount due to the entity within one year of the balance sheet date (or one operating cycle, if longer) from outside sources, including trade accounts receivable, notes and loans receivable, as well as any other types of receivables, net of allowances established for the purpose of reducing such receivables to an amount that approximates their net realizable value.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 3

-Subparagraph a

-Article 5

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 210.5-02.3,4)

-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 4

-Article 5

+ Details

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

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

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