Commitments and Contingencies - Commitments and Agreements (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended			6 Months Ended				6 Months Ended
	Sep. 30, 2013 Alabama Gas Corporation site	Jun. 30, 2014 Alabama Gas Corporation site	Jun. 30, 2014 Crude Oil and Natural Gas MBoe	Jun. 30, 2014 Natural Gas, Delivery and Storage Alabama Gas Corporation	Jun. 30, 2013 Natural Gas, Delivery and Storage Alabama Gas Corporation	Jun. 30, 2014 Natural Gas Alabama Gas Corporation Mcf	Jun. 30, 2014 Wholly Owned Properties Alabama Gas Corporation site	Jun. 30, 2014 Mitchell County, Texas [Member] pit
Long-term Purchase Commitment [Line Items]
Remaining contractual volume			6,000
Long-term purchase commitment				$ 140
Long-term purchase commitment, time period				September 2024		August 2020
Long-term commitments expense recognized in statement of operations				23.5	26.0
Long-term purchase commitment minimum quantity required						115,000,000
Recorded unconditional purchase obligation						0.3
Recorded unconditional purchase obligation, time period						December 2014
Recorded unconditional purchase obligation, market value						0.3
Chain of title, manufactured gas plant sites		9					4
Chain of title, manufactured gas distribution sites		5					1
Environmental matters, number of sites								9
Anticipated environmental costs								3.4
Environmental costs paid								1.9
Accrued environmental costs								0.5
Preliminary estimate of estimate for further remediation								$ 1.0
Number of residence that purportedly exceed certain risk levels for contamination	50