Employee Benefit Plans (Benefit Obligations) (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Amounts recognized in balance sheet
Noncurrent assets		$ 1,404,000	$ 0
Noncurrent liabilities		(110,282,000)	(62,532,000)
Amounts recognized to accumulated other comprehensive income:
Total accumulated other comprehensive income (loss)		52,507,000	38,584,000
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation		269,101,000	211,896,000
Projected benefit obligation:
Balance at beginning of period	233,772,000	250,619,000	233,772,000
Service cost		10,527,000	9,173,000	8,574,000
Interest cost		10,801,000	10,960,000	11,365,000
Actuarial (gain) loss		65,048,000	17,024,000
Plan amendments		0	(169,000)
Termination benefit charge	(400,000)	0	414,000	0
Retiree drug subsidy program		0	0
Benefits paid		(13,455,000)	(20,555,000)
Balance at end of period		323,540,000	250,619,000	233,772,000
Plan assets:
Fair value of plan assets at beginning of period	212,454,000	195,659,000	212,454,000
Actual return (loss) on plan assets		24,841,000	1,485,000
Employer contributions		2,379,000	2,275,000
Benefits paid		(13,455,000)	(20,555,000)
Fair value of plan assets at end of period		209,424,000	195,659,000	212,454,000
Funded status of plan
Funded status of plan		(114,116,000)	(54,960,000)
Amounts recognized in balance sheet
Noncurrent assets		0	0
Current liabilities		(3,834,000)	(2,371,000)
Noncurrent liabilities		(110,282,000)	(52,589,000)
Net asset (liability) recognized		(114,116,000)	(54,960,000)
Amounts recognized to accumulated other comprehensive income:
Prior service costs, net of taxes		528,000	749,000
Net actuarial (gain) loss, net of taxes		52,472,000	36,976,000
Transition obligation, net of taxes		0	0
Total accumulated other comprehensive income (loss)		53,000,000	37,725,000
Postretirement Benefit Plans
Projected benefit obligation:
Balance at beginning of period	83,748,000	88,064,000	83,748,000
Service cost		1,853,000	1,769,000	2,064,000
Interest cost		4,248,000	4,443,000	4,833,000
Actuarial (gain) loss		(5,413,000)	1,858,000
Plan amendments		0	0
Termination benefit charge		0	0
Retiree drug subsidy program		360,000	302,000
Benefits paid		(3,327,000)	(4,056,000)
Balance at end of period		85,785,000	88,064,000	83,748,000
Plan assets:
Fair value of plan assets at beginning of period	80,118,000	78,121,000	80,118,000
Actual return (loss) on plan assets		8,778,000	(1,653,000)
Employer contributions		3,617,000	3,712,000
Benefits paid		(3,327,000)	(4,056,000)
Fair value of plan assets at end of period		87,189,000	78,121,000	80,118,000
Funded status of plan
Funded status of plan		1,404,000	(9,943,000)
Amounts recognized in balance sheet
Noncurrent assets		1,404,000	0
Current liabilities		0	0
Noncurrent liabilities		0	(9,943,000)
Net asset (liability) recognized		1,404,000	(9,943,000)
Amounts recognized to accumulated other comprehensive income:
Prior service costs, net of taxes		0	0
Net actuarial (gain) loss, net of taxes		(715,000)	451,000
Transition obligation, net of taxes		222,000	408,000
Total accumulated other comprehensive income (loss)		(493,000)	859,000
Alabama Gas Corporation
Amounts recognized in balance sheet
Noncurrent assets		848,000	0
Noncurrent liabilities		(43,611,000)	(21,383,000)
Alabama Gas Corporation | Pension Plans
Amounts recognized to accumulated other comprehensive income:
Regulatory Assets		89,500,000	67,800,000
Alabama Gas Corporation | Postretirement Benefit Plans
Amounts recognized to accumulated other comprehensive income:
Regulatory Assets			8,400,000
Regulatory Liabilities		$ 1,200,000