(Net Periodic Benefit Cost) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2011	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Long-term Disability Plan [Member]
Components of net periodic benefit cost:
Expense related to long-term disability plan			$ 500,000	$ 400,000	$ 500,000
Pension Plans
Components of net periodic benefit cost:
Service cost			9,173,000	8,574,000	7,340,000
Interest cost			10,960,000	11,365,000	12,064,000
Expected long-term return on assets			(15,471,000)	(12,915,000)	(14,002,000)
Prior service cost amortization			496,000	496,000	579,000
Actuarial loss			6,435,000	5,773,000	3,987,000
Termination benefit charge	414,000	145,000	414,000	0	145,000
Net periodic expense			12,007,000	13,293,000	10,113,000
Postretirement Benefit Plans
Components of net periodic benefit cost:
Service cost			1,769,000	2,064,000	1,813,000
Interest cost			4,443,000	4,833,000	4,849,000
Expected long-term return on assets			(4,418,000)	(3,986,000)	(3,542,000)
Actuarial loss			0	0	228,000
Termination benefit charge			0	0
Transition amortization			1,917,000	1,917,000	1,917,000
Net periodic expense			3,711,000	4,828,000	5,265,000
Alabama Gas Corporation | Pension Plans
Components of net periodic benefit cost:
Net periodic expense			5,200,000	6,300,000	4,200,000
Alabama Gas Corporation | Postretirement Benefit Plans
Components of net periodic benefit cost:
Net periodic expense			$ 2,800,000	$ 3,600,000	$ 4,100,000