Financial Instruments (Details) (USD $)	3 Months Ended	9 Months Ended					9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2011 Swap Senior Loans	Sep. 30, 2013 Swap Senior Loans Level 1	Dec. 31, 2012 Swap Senior Loans Level 1	Dec. 31, 2011 Senior Term Loans, Due March 31, 2014 to November 29, 2016 Swap Senior Loans	Sep. 30, 2013 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation	Sep. 30, 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	Sep. 30, 2013 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure Alabama Gas Corporation	Dec. 31, 2012 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure Alabama Gas Corporation	Sep. 30, 2013 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Sep. 30, 2013 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure Alabama Gas Corporation
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value									$ 1,188,200,000	$ 1,255,800,000	$ 262,800,000	$ 284,700,000	$ 1,154,000,000	$ 250,000,000
Face amount						200,000,000
Interest rate			2.4175%
Interest rate cash flow hedge liability at fair value				2,100,000	3,300,000
Financing receivable							10,800,000	10,700,000
Financing receivable recorded investment average balance 0 to 60 days past due							3,000
Average finance receivable term							84 months
Time after which delinquent accounts are evaluated	90 days
Financing receivable recorded investment, equal to greater than 90 days past due							700,000	500,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses as of December 31, 2012		470,000
Provision		191,000
Allowance for credit losses as of September 30, 2013	$ 661,000	$ 661,000