Financial Instruments (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Sep. 30, 2012 Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Sep. 30, 2012 Alabama Gas Corporation	Dec. 31, 2011 Alabama Gas Corporation	Sep. 30, 2012 Alabama Gas Corporation Long-term Debt, Including Current Portion, Fair Value, Fair Value Disclosure	Sep. 30, 2012 Alabama Gas Corporation Long-term Debt, Including Current Portion, Carrying Value, Fair Value Disclosure	Dec. 31, 2011 Senior Loans [Member] Swap [Member]	Dec. 31, 2011 Senior Loans [Member] Swap [Member] Senior Term Loans, Due March 31, 2014 to November 29, 2016 [Member]	Sep. 30, 2012 Senior Loans [Member] Fair Value, Inputs, Level 1 [Member] Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value		$ 1,256,200,000	$ 1,154,100,000			$ 284,900,000	$ 250,100,000
Debt Instrument, Face Amount									200,000,000
Debt Instrument, Interest Rate, Stated Percentage								2.4175%
Derivative Asset, Fair Value, Gross Liability										3,700,000
Financing Receivable				10,700,000	10,500,000
Financing Receivable, Recorded Investment, Average Balance 0 to 60 Days Past Due				3,000
Average Finance Receivable Term				up to 60 months
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due				467,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses, beginning balance	421,000
Provision	21,000
Allowance for credit losses, ending balance	$ 442,000