Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2014 Maximum	Mar. 31, 2014 Senior Loans Swap	Mar. 31, 2014 Senior Loans Swap Level 2	Dec. 31, 2013 Senior Loans Swap Level 2	Mar. 31, 2014 Senior Loans Senior Term Loans, Due March 31, 2014 to November 29, 2016 Swap	Mar. 31, 2014 Alabama Gas Corporation	Dec. 31, 2013 Alabama Gas Corporation	Mar. 31, 2014 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Dec. 31, 2013 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Mar. 31, 2014 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure Alabama Gas Corporation	Dec. 31, 2013 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure Alabama Gas Corporation	Mar. 31, 2014 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Dec. 31, 2013 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Mar. 31, 2014 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure Alabama Gas Corporation	Dec. 31, 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									$ 1,421,200,000	$ 1,420,700,000	$ 263,500,000	$ 258,800,000	$ 1,388,900,000	$ 1,403,900,000	$ 249,900,000	$ 249,900,000
Face amount						200,000,000
Interest rate			2.6675%
Interest rate cash flow hedge liability at fair value				1,600,000	1,800,000
Financing receivable							10,500,000	10,800,000
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							300,000	400,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses as of December 31, 2013	423,000
Provision	(124,000)
Allowance for credit losses as of March 31, 2014	$ 299,000