Employee Benefit Plans (Benefit Obligations) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2011	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Amounts recognized in balance sheet
Noncurrent assets			$ 0	$ 13,907,000
Noncurrent liabilities			(62,532,000)	(36,551,000)
Amounts recognized to accumulated other comprehensive income:
Total accumulated other comprehensive income (loss)			38,584,000	30,730,000
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			211,896,000	196,421,000
Projected benefit obligation:
Balance at beginning of period	233,772,000		233,772,000	213,920,000
Service cost			9,173,000	8,574,000	7,340,000
Interest cost			10,960,000	11,365,000	12,064,000
Actuarial (gain) loss			17,024,000	12,961,000
Plan amendments			(169,000)	0
Termination benefit charge	414,000	145,000	414,000	0	145,000
Retiree drug subsidy program			0	0
Benefits paid			(20,555,000)	(13,048,000)
Balance at end of period			250,619,000	233,772,000	213,920,000
Plan assets:
Fair value of plan assets at beginning of period	212,454,000		212,454,000	167,653,000
Actual return (loss) on plan assets			1,485,000	20,443,000
Employer contributions			2,275,000	37,406,000
Benefits paid			(20,555,000)	(13,048,000)
Fair value of plan assets at end of period			195,659,000	212,454,000	167,653,000
Funded status of plan
Funded status of plan			(54,960,000)	(21,318,000)
Amounts recognized in balance sheet
Noncurrent assets			0	12,804,000
Current liabilities			(2,371,000)	(2,304,000)
Noncurrent liabilities			(52,589,000)	(31,818,000)
Net liability recognized			(54,960,000)	(21,318,000)
Amounts recognized to accumulated other comprehensive income:
Prior service costs, net of taxes			749,000	945,000
Net actuarial (gain) loss, net of taxes			36,976,000	30,112,000
Transition obligation, net of taxes			0	0
Total accumulated other comprehensive income (loss)			37,725,000	31,057,000
Postretirement Benefit Plans
Projected benefit obligation:
Balance at beginning of period	83,748,000		83,748,000	84,085,000
Service cost			1,769,000	2,064,000	1,813,000
Interest cost			4,443,000	4,833,000	4,849,000
Actuarial (gain) loss			1,858,000	(3,062,000)
Plan amendments			0	0
Termination benefit charge			0	0
Retiree drug subsidy program			302,000	0
Benefits paid			(4,056,000)	(4,172,000)
Balance at end of period			88,064,000	83,748,000	84,085,000
Plan assets:
Fair value of plan assets at beginning of period	80,118,000		80,118,000	72,227,000
Actual return (loss) on plan assets			(1,653,000)	7,580,000
Employer contributions			3,712,000	4,483,000
Benefits paid			(4,056,000)	(4,172,000)
Fair value of plan assets at end of period			78,121,000	80,118,000	72,227,000
Funded status of plan
Funded status of plan			(9,943,000)	(3,630,000)
Amounts recognized in balance sheet
Noncurrent assets			0	1,103,000
Current liabilities			0	0
Noncurrent liabilities			(9,943,000)	(4,733,000)
Net liability recognized			(9,943,000)	(3,630,000)
Amounts recognized to accumulated other comprehensive income:
Prior service costs, net of taxes			0	0
Net actuarial (gain) loss, net of taxes			451,000	(915,000)
Transition obligation, net of taxes			408,000	580,000
Total accumulated other comprehensive income (loss)			859,000	(335,000)
Alabama Gas Corporation
Amounts recognized in balance sheet
Noncurrent assets			0	9,201,000
Noncurrent liabilities			(21,383,000)	(4,733,000)
Alabama Gas Corporation | Pension Plans
Amounts recognized to accumulated other comprehensive income:
Regulatory Assets			67,800,000	54,200,000
Alabama Gas Corporation | Postretirement Benefit Plans
Amounts recognized to accumulated other comprehensive income:
Regulatory Assets			8,400,000	5,000,000
Regulatory Liabilities				$ 800,000