Financial Instruments and Risk Management (Financial Instruments and Finance Receivables) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses, beginning balance	$ 421,000	$ 447,000
Provision	49,000	(26,000)
Allowance for credit losses, ending balance	470,000	421,000
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	1,255,800,000	1,214,900,000
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	1,154,000,000	1,155,200,000
Alabama Gas Corporation
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Recorded Unconditional Purchase Obligation	300,000
Recorded Unconditional Purchase Obligation, Market Value	300,000
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	500,000	400,000
Alabama Gas Corporation | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	284,700,000	274,900,000
Alabama Gas Corporation | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 300,000,000	$ 300,000,000