Commitments and Contingencies - Commitments and Agreements (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				3 Months Ended
	Sep. 30, 2013 Alabama Gas Corporation site	Mar. 31, 2014 Alabama Gas Corporation site	Mar. 31, 2014 Alabama Gas Corporation Wholly Owned Properties site	Mar. 31, 2014 Crude Oil and Natural Gas MBoe	Mar. 31, 2014 Natural Gas, Delivery and Storage Alabama Gas Corporation	Mar. 31, 2013 Natural Gas, Delivery and Storage Alabama Gas Corporation	Mar. 31, 2014 Natural Gas Alabama Gas Corporation Mcf
Long-term Purchase Commitment [Line Items]
Remaining contractual volume				6,400
Long-term purchase commitment					$ 150
Long-term purchase commitment, time period					September 2024		August 2020
Long-term commitments expense recognized in statement of operations					13.4	14.4
Long-term purchase commitment minimum quantity required							123,000,000
Recorded unconditional purchase obligation, time period							December 2014
Recorded unconditional purchase obligation							$ 0.3
Chain of title, manufactured gas plant sites		9	4
Chain of title, manufactured gas distribution sites		5	1
Number of sites with removal activities	50