Accumulated Other Comprehensive Income (Loss) Reclassifications of Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenue	$ 472,733,000	$ 320,406,000	$ 471,495,000	$ 474,016,000	$ 414,297,000	$ 276,429,000	$ 451,904,000	$ 398,189,000	$ 1,738,650,000		$ 1,540,819,000	$ 1,373,113,000
Interest expense									(69,200,000)		(65,542,000)	(44,822,000)
Income From Continuing Operations									193,147,000		255,220,000	224,305,000
Income tax expense									(105,282,000)		(144,534,000)	(126,322,000)
Pension and postretirement plans:
Settlement charges		64,000		500,000
Settlement charges expensed		18,000		100,000
Alabama Gas Corporation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenue	142,771,000	48,368,000	104,514,000	237,685,000	124,406,000	61,809,000	70,887,000	194,487,000
Interest expense									(15,649,000)		(16,284,000)	(14,740,000)
Income tax expense									(34,687,000)		(30,244,000)	(26,670,000)
Pension and postretirement plans:
Settlements recognized as a pension asset		46,000		400,000
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									12,324,000
Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income From Continuing Operations									33,961,000
Income tax expense									(12,957,000)
Net of tax									21,004,000
Pension and Postretirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income From Continuing Operations									(13,354,000)
Income tax expense									4,674,000
Net of tax									(8,680,000)
Pension and postretirement plans:
Transition obligation									(319,000)
Prior service cost									(257,000)
Actuarial losses									(12,357,000)	[1]
Actuarial losses on settlement charges									(421,000)
Commodity Contract | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenue									35,684,000
Interest Rate Swap | Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									$ (1,723,000)

[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. In the third quarter of 2013, the Company incurred a settlement charge of $64,000 for the payment of lump sums from the nonqualified supplemental retirement plans, of which $18,000 is recognized in actuarial losses above and $46,000 is recognized as a regulatory asset at Alagasco and reported in actuarial losses on settlement charges above.