Employee Benefit Plans (Net Periodic Benefit Cost) (Details) (USD $)	3 Months Ended				12 Months Ended									3 Months Ended		12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Mar. 31, 2013	Mar. 31, 2011	Dec. 31, 2013 Long-term Disability Plan	Dec. 31, 2012 Long-term Disability Plan	Dec. 31, 2011 Long-term Disability Plan	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	Sep. 30, 2013 Alabama Gas Corporation	Mar. 31, 2013 Alabama Gas Corporation	Dec. 31, 2013 Alabama Gas Corporation Pension Plans	Dec. 31, 2012 Alabama Gas Corporation Pension Plans	Dec. 31, 2011 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, 2011 Alabama Gas Corporation Postretirement Benefit Plans
Components of net periodic benefit cost:
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
Expected long-term return on assets								(14,731,000)	(14,093,000)	(15,471,000)	(5,024,000)	(4,438,000)	(4,418,000)
Prior service cost amortization								490,000	517,000	496,000
Actuarial loss amortization								13,979,000	8,603,000	6,435,000	(120,000)	37,000	0
Termination benefit charge				(400,000)				0	0	414,000
Settlement charge	800,000							1,373,000	0	0
Transition obligation amortization											1,296,000	1,917,000	1,917,000
Curtailment gain											(1,229,000)	0	0
Net periodic expense								26,523,000	16,355,000	12,007,000	121,000	3,617,000	3,711,000			12,100,000	7,800,000	5,200,000	800,000	2,700,000	2,800,000
Settlement charges		64,000	500,000
Settlement charges expensed		18,000	100,000
Settlements recognized as a pension asset														46,000	400,000
Expense related to long-term disability plan					$ 600,000	$ 700,000	$ 500,000