Proposed Merger with Nicor (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2011
Notes To Financial Statements [Abstract]
Transaction expenses associated with merger	$ 8	$ 26
Transaction expenses associated with merger, net of tax	5	16
Date entered into merger agreement with Nicor		December 6, 2010
Exchange ratio of Nicor stock to AGL Stock (in hundredths)		0.8382
Cash payment per share of Nicor Stock (in dollars per share)		$ 21.20
AGL shareholders expected ownership percentage of the combined company		0.67
Nicor shareholders expected ownership percentage of the combined company		0.33
Merger Termination Fee under specified circumstances		67
Merger Termination Fee Due To Failure To Obtain Financing		$ 115

Upon closing of the proposed merger, AGL shareholders ownership percentage of the combined company.

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Upon closing of the proposed merger, Nicor shareholders ownership percentage of the combined company.

+ Details

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Expenses and losses, after applicable taxes, for each transaction with the acquiree that was recognized separately from the acquisition of assets and assumptions of liabilities in the business combination.

+ Details

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Cash payment per share of Nicor Stock (in dollars per share) Merger consideration

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- Definition

The Merger Agreement with Nicor contains termination rights for both us and Nicor and provides that, if we terminate the agreement under specified circumstances, we may be required to pay a termination fee.

+ Details

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Additional termination fee required to pay if we terminate the merger agreement due to a failure to obtain the necessary financing for the transaction.

+ Details

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- Details

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Exchange ratio of Nicor stock to AGL Stock (in hundredths) Merger consideration

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Month, day and year when the business acquisition agreement was executed.

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The expenses and losses recorded for each transaction with the acquiree that was recognized separately from the acquisition of assets and assumptions of liabilities in the business combination.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6909870&loc=d3e1392-128463

-Subparagraph (e)(3)

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