Financial Instruments (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Alabama Gas Corporation
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financing receivable	$ 10,600,000	$ 10,600,000	$ 10,800,000
Financing receivable recorded investment average balance	3,000	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	300,000	400,000
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses as of December 31, 2013		423,000
Provision		(118,000)
Allowance for credit losses as of June 30, 2014	305,000	305,000
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	1,148,700,000	1,148,700,000	1,161,900,000
Fair Value | Alabama Gas Corporation
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	266,600,000	266,600,000	258,800,000
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	1,124,000,000	1,124,000,000	1,154,000,000
Reported Value Measurement | Alabama Gas Corporation
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt fair value	$ 249,800,000	$ 249,800,000	$ 249,900,000