Summary of Significant Accounting Policies (Details) (Alabama Gas Corporation, USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2010
Overstatement to allowance for doubtful accounts
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Overstatement as result of error	$ 3.0
Overstatement of net income
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Overstatement as result of error	$ 0.6
Earnings per diluted share
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Overstatement, (per diluted share) as result of error	$ 0.01