Reclassifications of Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Gains and (losses) on cash flow hedges:
Operating revenues	$ 492,679,000		$ 418,444,000
Interest expense	(16,754,000)		(15,425,000)
Income tax expense	(33,579,000)		(33,258,000)
Pension and postretirement plans:
Settlement charges	500,000
Settlement charges expensed	100,000
Alabama Gas Corporation
Gains and (losses) on cash flow hedges:
Interest expense	(4,030,000)		(4,062,000)
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	8,646,000
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	16,881,000
Income tax expense	(6,427,000)
Net of tax	10,454,000
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	(2,781,000)
Income tax expense	973,000
Net of tax	(1,808,000)
Pension and postretirement plans:
Transition obligation	(74,000)
Prior service cost	(78,000)
Actuarial losses	(2,254,000)	[1]
Actuarial losses on settlement charges	(375,000)	[1]
Commodity contracts | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Operating revenues	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	$ (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 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.