Commitments and Contingencies - Commitments and Agreements (Details) (USD $) In Millions, unless otherwise specified	2 Months Ended		9 Months Ended		9 Months Ended
	Jan. 15, 2012 agreement	Sep. 30, 2013 Contract Termination	Sep. 30, 2013 Alabama Gas Corporation site	Sep. 30, 2013 Crude Oil and Natural Gas MBoe	Sep. 30, 2013 Natural Gas, Delivery and Storage Alabama Gas Corporation	Sep. 30, 2013 Natural Gas Alabama Gas Corporation Mcf	Sep. 30, 2013 Wholly Owned Properties Alabama Gas Corporation site
Long-term Purchase Commitment [Line Items]
Remaining contractual volume				7,500
Number of agreements to secure drilling rigs	3
Maximum loss contingency range of possible loss		$ 14
Long-term purchase commitment					185
Long-term purchase commitment, time period					September 2024	August 2020
Long-term commitments expense recognized in statement of operations					36.8
Long-term purchase commitment minimum quantity required						146,000,000
Recorded unconditional purchase obligation, time period						October 2014
Recorded unconditional purchase obligation						0.8
Recorded unconditional purchase obligation, market value						$ 0.8
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