Employee Benefit Plans (Benefit Obligations) (Details) (USD $)					12 Months Ended												3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Pension Plans	Dec. 31, 2012 Pension Plans	Dec. 31, 2011 Pension Plans	Dec. 31, 2013 Postretirement Benefit Plans	Dec. 31, 2012 Postretirement Benefit Plans	Dec. 31, 2011 Postretirement Benefit Plans	Dec. 31, 2013 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation	Dec. 31, 2013 Alabama Gas Corporation Pension Plans	Dec. 31, 2012 Alabama Gas Corporation Pension Plans	Dec. 31, 2013 Alabama Gas Corporation Postretirement Benefit Plans	Dec. 31, 2012 Alabama Gas Corporation Postretirement Benefit Plans	Dec. 31, 2013 Black Warrior Basin
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation					$ 253,030,000	$ 269,101,000
Projected benefit obligation:
Balance at beginning of period					323,540,000	250,619,000		85,785,000	88,064,000
Service cost					14,173,000	10,527,000	9,173,000	1,694,000	1,853,000	1,769,000
Interest cost					11,239,000	10,801,000	10,960,000	3,504,000	4,248,000	4,443,000
Actuarial (gain) loss					(28,339,000)	65,048,000		(21,681,000)	(5,413,000)
Curtailment gain					(4,223,000)	0		(1,255,000)	0								(1,200,000)
Retiree drug subsidy program					0	0		261,000	360,000
Benefits paid					(23,036,000)	(13,455,000)		(4,726,000)	(3,327,000)
Balance at end of period					293,354,000	323,540,000	250,619,000	63,582,000	85,785,000	88,064,000
Plan assets:
Fair value of plan assets at beginning of period			17,399,000	26,841,000	209,424,000	195,659,000		87,189,000	78,121,000
Actual return on plan assets					22,977,000	24,841,000		14,892,000	8,778,000
Employer contributions					10,169,000	2,379,000		1,578,000	3,617,000
Benefits paid					(23,036,000)	(13,455,000)		(4,726,000)	(3,327,000)
Fair value of plan assets at end of period			17,399,000	26,841,000	219,534,000	209,424,000	195,659,000	98,933,000	87,189,000	78,121,000
Funded status of plan
Funded status of plans					(73,820,000)	(114,116,000)		35,351,000	1,404,000
Amounts recognized in balance sheet
Noncurrent assets	35,351,000	1,404,000			0	0		35,351,000	1,404,000		26,457,000	848,000
Current liabilities					(6,145,000)	(3,834,000)		0	0
Noncurrent liabilities	(67,675,000)	(110,282,000)			(67,675,000)	(110,282,000)		0	0		(20,191,000)	(43,611,000)
Net asset (liability) recognized					(73,820,000)	(114,116,000)		35,351,000	1,404,000
Amounts recognized to accumulated other comprehensive income:
Prior service costs, net of taxes					323,000	528,000		0	0
Net actuarial (gain) loss, net of taxes					37,479,000	52,472,000		(5,584,000)	(715,000)
Transition obligation, net of taxes					0	0		27,000	222,000
Total accumulated other comprehensive income (loss)	32,245,000	52,507,000			37,802,000	53,000,000		(5,557,000)	(493,000)
Regulatory Assets													58,200,000	89,500,000
Regulatory Liabilities															$ 26,200,000	$ 1,200,000