Reclassifications of Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Gains and (losses) on cash flow hedges:
Operating revenues from continuing operations	$ 490,057,000		$ 470,355,000	$ 982,736,000		$ 888,799,000
Interest expense	(17,306,000)		(15,835,000)	(34,060,000)		(31,260,000)
Income tax expense	(46,551,000)		(73,553,000)	(80,130,000)		(106,811,000)
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	(3,833,000)		(3,959,000)	(7,863,000)		(8,021,000)
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	1,054,000			9,699,000
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	4,077,000			20,956,000
Income tax expense	(1,562,000)			(7,989,000)
Net of tax	2,515,000			12,967,000
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Total	(2,249,000)			(5,029,000)
Income tax expense	788,000			1,761,000
Net of tax	(1,461,000)			(3,268,000)
Pension and postretirement plans:
Transition obligation	(74,000)			(147,000)
Prior service cost	(78,000)			(157,000)
Actuarial losses	(2,097,000)	[1]		(4,350,000)	[1]
Actuarial losses on settlement charges	0	[1]		(375,000)	[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	4,503,000			21,792,000
Interest rate swap | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Gains and (losses) on cash flow hedges:
Interest expense	$ (426,000)			$ (836,000)

[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.