Commitments and Contingencies (Environmental Matters) (Details) (Alabama Gas Corporation, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012 site
Site Contingency [Line Items]
Chain of title, manufactured gas plant sites	9
Chain of title, manufactured gas distribution sites	5
Cost incurred, environmental redediation	$ 5
Sites owned by Alagasco
Site Contingency [Line Items]
Chain of title, manufactured gas plant sites	4
Chain of title, manufactured gas distribution sites	1

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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