Gallons of diesel fuel purchased annually.

No definition available.

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Namespace Prefix:	aci_
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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:	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

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Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
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- Definition

The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

+ References

Reference 1: 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

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- Details

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- 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

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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

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph b(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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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 Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44C

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205H

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: 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

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Derivatives (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Derivative [Line Items]
Current asset for the right to reclaim cash collateral	$ 4.7		$ 4.7		$ 16.2
Net unrealized and realized gains (losses) related to trading portfolio	4.9	4.6	2.1	0.7
Gain on derivative contracts designated as hedge instruments in cash flow hedges to be reclassified from OCI into earnings during the next twelve months	1.6		1.6
Minimum Purchase [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually			57,000,000
Maximum Purchase [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually			67,000,000
Year One [Member]
Derivative [Line Items]
Value of trading portfolio realized			0.4
Year Two [Member]
Derivative [Line Items]
Value of trading portfolio realized			$ 5.9
Heating Oil-Diesel Purchases [Member]
Derivative [Line Items]
Quantities under derivative contracts	74,000,000		74,000,000
Heating Oil-Diesel Purchases [Member] \| Year One [Member]
Derivative [Line Items]
Percent of expected requirements covered	94.00%		94.00%
Heating Oil-Diesel Purchases [Member] \| Year Two [Member]
Derivative [Line Items]
Percent of expected requirements covered	73.00%		73.00%
Heating Oil-Fuel Surchages [Member]
Derivative [Line Items]
Quantities under derivative contracts	4,000,000		4,000,000
Heating Oil-Fuel Surchages [Member] \| Year One [Member]
Derivative [Line Items]
Percent of expected requirements covered	47.00%		47.00%
Heating Oil-Fuel Surchages [Member] \| Year Two [Member]
Derivative [Line Items]
Percent of expected requirements covered	26.00%		26.00%