Financial Instruments (Details) (USD $)	3 Months Ended															3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Dec. 31, 2012 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Mar. 31, 2013 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Dec. 31, 2012 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Mar. 31, 2013 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation	Mar. 31, 2013 Alabama Gas Corporation Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Dec. 31, 2012 Alabama Gas Corporation Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Mar. 31, 2013 Alabama Gas Corporation Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Dec. 31, 2012 Alabama Gas Corporation Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Dec. 31, 2011 Senior Loans Swap	Dec. 31, 2011 Senior Loans Swap Senior Term Loans, Due March 31, 2014 to November 29, 2016	Mar. 31, 2013 Senior Loans Level 1 Swap	Dec. 31, 2012 Senior Loans Level 1 Swap	Mar. 31, 2013 Maximum Alabama Gas Corporation
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value		$ 1,216,900,000	$ 1,255,800,000	$ 1,154,000,000	$ 1,154,000,000			$ 271,300,000	$ 284,700,000	$ 250,000,000	$ 250,000,000
Face amount													200,000,000
Interest rate												2.4175%
Interest rate cash flow hedge liability at fair value														2,900,000	3,300,000
Financing receivable						10,400,000	10,700,000
Financing receivable recorded investment average balance 0 to 60 days past due						3,000
Average finance receivable term																60 months
Time after which delinquent accounts are evaluated	90 days
Time period after which accounts are written off	12 months
Financing receivable recorded investment, equal to greater than 90 days past due						600,000	500,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses as of December 31, 2012	470,000
Provision	116,000
Allowance for credit losses as of March 31, 2013	$ 586,000