Employee Benefit Plans (Net Periodic Benefit Cost) (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Long-term Disability Plan
Components of net periodic benefit cost:
Expense related to long-term disability plan		$ 700,000	$ 500,000	$ 400,000
Pension Plans
Components of net periodic benefit cost:
Service cost		10,527,000	9,173,000	8,574,000
Interest cost		10,801,000	10,960,000	11,365,000
Expected long-term return on assets		(14,093,000)	(15,471,000)	(12,915,000)
Prior service cost amortization		517,000	496,000	496,000
Defined Benefit Plan, Amortization of Gains (Losses)		8,603,000	6,435,000	5,773,000
Termination benefit charge	(400,000)	0	414,000	0
Net periodic expense		16,355,000	12,007,000	13,293,000
Postretirement Benefit Plans
Components of net periodic benefit cost:
Service cost		1,853,000	1,769,000	2,064,000
Interest cost		4,248,000	4,443,000	4,833,000
Expected long-term return on assets		(4,438,000)	(4,418,000)	(3,986,000)
Defined Benefit Plan, Amortization of Gains (Losses)		37,000	0	0
Termination benefit charge		0	0
Transition obligation amortization		1,917,000	1,917,000	1,917,000
Net periodic expense		3,617,000	3,711,000	4,828,000
Alabama Gas Corporation | Pension Plans
Components of net periodic benefit cost:
Net periodic expense		7,800,000	5,200,000	6,300,000
Alabama Gas Corporation | Postretirement Benefit Plans
Components of net periodic benefit cost:
Net periodic expense		$ 2,700,000	$ 2,800,000	$ 3,600,000