Commitments and Contingencies - Commitments and Agreements (Details) (USD $) In Millions, unless otherwise specified	2 Months Ended				6 Months Ended
	Jan. 15, 2012 agreement	Jun. 30, 2013 Contract Termination	Jun. 30, 2013 Alabama Gas Corporation site	Jun. 30, 2013 Crude Oil and Natural Gas MBoe	Jun. 30, 2013 Natural Gas, Delivery and Storage Alabama Gas Corporation	Jun. 30, 2012 Natural Gas, Delivery and Storage Alabama Gas Corporation	Jun. 30, 2013 Natural Gas Alabama Gas Corporation Mcf	Jun. 30, 2013 Wholly Owned Properties Alabama Gas Corporation site
Long-term Purchase Commitment [Line Items]
Remaining contractual volume				8,200
Number of agreements to secure drilling rigs	3
Maximum loss contingency range of possible loss		$ 16
Long-term purchase commitment					83
Long-term purchase commitment, time period					September 2024		August 2020
Long-term commitments expense recognized in statement of operations					26.0	25.9
Long-term purchase commitment minimum quantity required							155,000,000
Recorded unconditional purchase obligation, time period							March 2014
Recorded unconditional purchase obligation							1.0
Recorded unconditional purchase obligation, market value							$ 1.0
Chain of title, manufactured gas plant sites			9					4
Chain of title, manufactured gas distribution sites			5					1