Gallons of diesel fuel purchased annually.

No definition available.

Name:	aci_AnnualPurchaseOfCommodityForOperations
Namespace Prefix:	aci_
Data Type:	num:volumeItemType
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

Name:	aci_ValueOfTradingPortfolioRealized
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226016-175313

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (d)(2)

-URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959

+ Details

Name:	us-gaap_DerivativeCollateralObligationToReturnCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

+ References

+ Details

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

- Details

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

- Definition

The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need.

+ References

No definition available.

+ Details

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

- Definition

The estimated net amount of unrealized gains or losses on price risk cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7668309&loc=d3e80748-113994

+ Details

Name:	us-gaap_PriceRiskCashFlowHedgeUnrealizedGainLossToBeReclassifiedDuringNext12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The net gain (loss) on trading activities (including both derivative and nonderivative instruments) recognized in the statement of financial performance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4F

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624186-113959

+ Details

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

Derivatives (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Derivative [Line Items]
Current liability for the obligation to return cash collateral	$ 1.9		$ 1.9
Current asset for the right to reclaim cash collateral					2.2
Net unrealized and realized gains (losses) related to trading portfolio	3.1	4.9	3.5	2.1
Gain on derivative contracts designated as hedge instruments in cash flow hedges to be reclassified from OCI into earnings during the next twelve months	0.8		0.8
Year One [Member]
Derivative [Line Items]
Value of trading portfolio realized			6.3
Year Two [Member]
Derivative [Line Items]
Value of trading portfolio realized			$ 2.9
Diesel Purchases [Member] \| Year One, Remainder [Member]
Derivative [Line Items]
Percent of expected requirements covered	88.00%		88.00%
Diesel Purchases [Member] \| Year Two, Through End of Third Quarter [Member]
Derivative [Line Items]
Percent of expected requirements covered	80.00%		80.00%
Diesel Purchases [Member] \| Heating Oil [Member]
Derivative [Line Items]
Quantities under derivative contracts	70,000,000		70,000,000
Diesel Purchases [Member] \| Minimum Annual Purchase [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually			57,000,000
Diesel Purchases [Member] \| Maximum [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually			67,000,000