Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning	$ 107,340	$ 97,415	$ 88,298
Liabilities incurred	3,994	4,627	4,033
Liabilities settled	(845)	(1,539)	(1,094)
Accretion expense	7,534	6,837	6,178
Balance of ARO, end	118,023	107,340	97,415
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Noncurrent	110,566	95,633
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, end	24,900	20,800
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Noncurrent	110,566	95,633
Alabama Gas Corporation \| Asset removal costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Noncurrent	$ 3,322	$ 994

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 143

-Paragraph 3, 10, 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 Accounting Standards Codification

-Topic 410

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-Section 50

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

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Amount of asset retirement obligations incurred during the period.

+ References

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

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-Name Accounting Standards Codification

-Topic 410

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-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.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

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-Section 50

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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-Publisher FASB

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-Number 143

-Paragraph 22

-Subparagraph c(2)

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

-Paragraph 9, 10

-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 Accounting Standards Codification

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Accretion expense of asset retirement obligations related to oil and gas producing activities.

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