Regulatory Assets and Liabilities (Details) (Alabama Gas Corporation, USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	$ 2,316	$ 2,756
Regulatory assets, noncurrent	91,857	84,890
Regulatory liability, current	64,401	49,006
Regulatory liability, noncurrent	82,580	94,125
RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	18,578	4,690
Regulatory liability, noncurrent	0	0
Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	6,445	28,504
Regulatory liability, noncurrent	0	0
Postretirement liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	0	0
Regulatory liability, noncurrent	26,948	26,197
Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	25,985	0
Regulatory liability, noncurrent	0	0
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	13,360	15,779
Regulatory liability, noncurrent	26,760	39,663
Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	0	0
Regulatory liability, noncurrent	28,152	27,528
Other
Regulatory Assets and Liabilities [Line Items]
Regulatory liability, current	33	33
Regulatory liability, noncurrent	720	737
Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	1,284	325
Regulatory assets, noncurrent	65,627	58,243
Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	0	0
Regulatory assets, noncurrent	18,825	18,046
Rate recovery of asset removal costs, net
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	0	0
Regulatory assets, noncurrent	3,405	4,601
Enhanced Stability Reserve [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	0	0
Regulatory assets, noncurrent	4,000	4,000
Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	0	2,406
Regulatory assets, noncurrent	0	0
RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory assets, current	1,032	25
Regulatory assets, noncurrent	$ 0	$ 0