Long-Term Debt and Notes Payable (Lines of Credit) (Details) (USD $)			12 Months Ended		0 Months Ended		12 Months Ended			0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Energen Corporation	Dec. 31, 2011 Energen Corporation	Oct. 30, 2012 Energen Corporation Syndicated Credit Facility	Oct. 29, 2010 Energen Corporation Syndicated Credit Facility	Dec. 31, 2012 Alabama Gas Corporation	Dec. 31, 2011 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation APSC Authorized Short-term Line of Credit	Oct. 30, 2012 Alabama Gas Corporation Syndicated Credit Facility	Oct. 29, 2010 Alabama Gas Corporation Syndicated Credit Facility
Line of Credit Facility [Line Items]
Maximum consolidated debt to capitalization ratio	65.00%						65.00%
Cross default provision, minimum debt default amount	$ 50,000,000						$ 50,000,000
Line of Credit Facility [Abstract]
Credit facilities	1,350,000,000	1,019,000,000			1,250,000,000	850,000,000				100,000,000	150,000,000
Credit facilities term (in years)					5 years	3 years				5 years	3 years
Short-term line of credit, authorization to borrow									200,000,000
Notes payable to banks	643,000,000	15,000,000	566,000,000	0			77,000,000	15,000,000
Available for borrowings	707,000,000	1,004,000,000
Maximum amount outstanding at any month-end			643,000,000	363,000,000			77,000,000	70,000,000
Average daily amount outstanding			$ 331,068,000	$ 229,094,000			$ 21,254,000	$ 29,268,000
Average daily amount outstanding, weighted average interest rates			1.82%	2.04%			1.44%	1.72%
Amount outstnding at year-end, weighted average interest rates			1.35%	3.57%			1.11%	3.57%