Reclassifications of Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Mar. 31, 2013	Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Gains and (losses) on cash flow hedges:
Operating revenues from continuing operations	$ 320,406,000			$ 276,429,000	$ 1,265,917,000		$ 1,126,522,000
Interest expense	(17,689,000)			(17,195,000)	(51,751,000)		(48,447,000)
Income tax expense	3,627,000			322,000	(73,897,000)		(110,508,000)
Pension and postretirement plans:
Settlement charges	64,000		500,000
Settlement charges expensed	100,000
Settlement charge expensed	18,000
Alabama Gas Corporation
Gains and (losses) on cash flow hedges:
Interest expense	(3,869,000)			(4,164,000)	(11,733,000)		(12,185,000)
Pension and postretirement plans:
Settlements recognized as a pension asset	400,000
Recognized as regulatory asset	46,000				46,000
Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Net of tax	3,488,000				13,186,000
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	7,987,000				28,944,000
Income tax expense	(3,048,000)				(11,037,000)
Net of tax	4,939,000				17,907,000
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	(2,234,000)				(7,263,000)
Income tax expense	783,000				2,542,000
Net of tax	(1,451,000)				(4,721,000)
Pension and postretirement plans:
Transition obligation	(74,000)				(220,000)
Prior service cost	(78,000)				(235,000)
Actuarial losses	(2,036,000)	[1]			(6,387,000)	[1]
Actuarial losses on settlement charges	(46,000)	[1]			(421,000)	[1]
Commodity contracts | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Operating revenues from continuing operations	8,433,000				30,226,000
Interest rate swap | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Interest expense	$ (446,000)				$ (1,282,000)

[1] In the first quarter of 2013, the Company incurred a settlement charge of $0.5 million for the payment of lump sums from the nonqualified supplemental retirement plans, of which $0.1 million is recognized in actuarial losses above and $0.4 million is recognized as a regulatory asset at Alagasco and reported in actuarial losses on settlement charges above.