Employee Benefit Plans (Details) (USD $)	3 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2013 pension_plan	Mar. 31, 2013	Jan. 31, 2013 Pension Plans	Sep. 30, 2013 Pension Plans	Sep. 30, 2012 Pension Plans	Sep. 30, 2013 Pension Plans	Sep. 30, 2012 Pension Plans	Sep. 30, 2013 Postretirement Benefit Plans	Sep. 30, 2012 Postretirement Benefit Plans	Sep. 30, 2013 Postretirement Benefit Plans	Sep. 30, 2012 Postretirement Benefit Plans	Sep. 30, 2013 Alabama Gas Corporation
Defined Benefit Plan Disclosure
Number of defined benefit non-contributory pension plans and certain nonqualified supplemental pension plans	2
Components of net periodic benefit cost:
Service cost				$ 3,602,000	$ 2,632,000	$ 10,806,000	$ 7,895,000	$ 444,000	$ 463,000	$ 1,333,000	$ 1,390,000
Interest cost				2,725,000	2,700,000	8,161,000	8,101,000	869,000	1,062,000	2,605,000	3,186,000
Expected long-term return on assets				(3,713,000)	(3,563,000)	(11,139,000)	(10,689,000)	(1,242,000)	(1,109,000)	(3,727,000)	(3,328,000)
Actuarial loss				3,597,000	2,099,000	10,962,000	6,297,000	0	9,000	0	27,000
Prior service cost amortization				123,000	129,000	367,000	388,000
Settlement charge				17,000	0	161,000	0
Transition amortization								324,000	479,000	973,000	1,438,000
Net periodic expense				6,351,000	3,997,000	19,318,000	11,992,000	395,000	904,000	1,184,000	2,713,000
Estimated required future employer contributions in current fiscal year				0		0
Contributions by employer in current fiscal year			9,000,000					400,000		1,200,000
Benefit payments made in current fiscal year				200,000		1,100,000
Estimated future benefit payments in current fiscal year						0
Settlement charges	64,000	500,000
Settlement charges expensed	100,000
Settlements recognized as a pension asset												400,000
Estimated future employer contributions in current fiscal year										400,000
Settlement charge expensed	18,000
Recognized as regulatory asset												$ 46,000