Reclassifications of Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Pension and Postretirement Plans Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Pension and Postretirement Plans Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Pension and Postretirement Plans Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Pension and Postretirement Plans Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Commodity contracts Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Commodity contracts Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Commodity contracts Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Commodity contracts Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Interest rate swap Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Interest rate swap Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Interest rate swap Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2013 Interest rate swap Cash Flow Hedges Reclassification out of Accumulated Other Comprehensive Income	Sep. 30, 2014 Supplemental Employee Retirement Plan	Mar. 31, 2014 Supplemental Employee Retirement Plan	Sep. 30, 2013 Supplemental Employee Retirement Plan	Mar. 31, 2013 Supplemental Employee Retirement Plan	Sep. 30, 2013 Alabama Gas Corporation Supplemental Employee Retirement Plan	Mar. 31, 2013 Alabama Gas Corporation Supplemental Employee Retirement Plan
Gains (losses) on cash flow hedges:
Reclassification Adjustment from AOCI on Derivatives																	$ 5,992,000	$ 8,433,000	$ 15,781,000	$ 30,226,000	$ (247,000)	$ (446,000)	$ (2,280,000)	$ (1,282,000)
Total									5,745,000	7,987,000	13,501,000	28,944,000	(1,502,000)	(2,234,000)	(11,871,000)	(7,263,000)
Income tax expense	(16,055,000)	(3,128,000)	(23,287,000)	(53,401,000)					(2,191,000)	(3,048,000)	(5,199,000)	(11,037,000)	525,000	783,000	4,155,000	2,542,000
Net of tax					2,577,000	3,488,000	586,000	13,186,000	3,554,000	4,939,000	8,302,000	17,907,000	(977,000)	(1,451,000)	(7,716,000)	(4,721,000)
Pension and postretirement plans:
Transition obligation													2,000	(74,000)	(17,000)	(220,000)
Prior service cost													(74,000)	(78,000)	(222,000)	(235,000)
Actuarial losses													(1,060,000)	(2,036,000)	(3,640,000)	(6,387,000)
Actuarial losses on settlement charges														0	(7,992,000)	0
Actuarial losses on settlement charges													(370,000)	(46,000)	0	(421,000)
Settlement charges																									0	400,000	64,000	500,000
Settlement charges expensed																												100,000
Settlements recognized as a pension asset																													46,000	400,000
Recognized in actuarial losses																											$ 18,000