Commitments and Contingencies (Environmental Matters) (Details) (Alabama Gas Corporation, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011 site	Sep. 30, 2011 Enhanced stability reserve	Sep. 29, 2011 Enhanced stability reserve	Dec. 31, 2011 Sites owned by Alagasco site
Site Contingency [Line Items]
Chain of title, manufactured gas plant sites	9			4
Chain of title, manufactured gas distribution sites	5			1
Cost incurred, environmental redediation	$ 4.8
Regulatory assets		$ 0	$ 4.8

The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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+ Details