Reclassifications of Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Gains and (losses) on cash flow hedges:
Operating revenues from continuing operations	$ 561,178,000		$ 474,016,000
Interest expense	(17,640,000)		(16,752,000)
Income tax expense	(32,797,000)		(32,409,000)
Alabama Gas Corporation
Gains and (losses) on cash flow hedges:
Interest expense	(3,965,000)		(4,030,000)
Non-Qualified Supplemental Retirement Plans
Pension and postretirement plans:
Settlement charges	400,000		500,000
Settlement charges expensed			100,000
Non-Qualified Supplemental Retirement Plans | Alabama Gas Corporation
Pension and postretirement plans:
Settlements recognized as a pension asset			400,000
Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Net of tax	(3,389,000)		8,646,000
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	3,608,000		16,881,000
Income tax expense	(1,384,000)		(6,427,000)
Net of tax	2,224,000		10,454,000
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	(8,636,000)		(2,781,000)
Income tax expense	3,023,000		973,000
Net of tax	(5,613,000)		(1,808,000)
Pension and postretirement plans:
Transition obligation	(10,000)		(74,000)
Prior service cost	(74,000)		(78,000)
Actuarial losses	(1,290,000)	[1]	(2,254,000)	[1]
Actuarial losses on settlement charges	(7,262,000)		0
Actuarial losses on settlement charges	0	[1]	(375,000)	[1]
Derivative commodity instruments | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Operating revenues from continuing operations	4,054,000		17,290,000
Interest rate swap | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Interest expense	$ (446,000)		$ (409,000)

[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 non-qualified supplemental retirement plans, of which $0.1 million is recognized in actuarial gains (losses) above and $0.4 million is recognized as a regulatory asset at Alagasco and reported in actuarial losses on settlement charges above.