Derivative Commodity Instruments - Derivatives Measured at Fair Value on a Recurring Basis (Details) (USD $)							3 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Level 2 Fair Value, Measurements, Recurring		Dec. 31, 2012 Level 2 Fair Value, Measurements, Recurring		Sep. 30, 2013 Level 3	Sep. 30, 2013 Level 3 Fair Value, Measurements, Recurring		Dec. 31, 2012 Level 3 Fair Value, Measurements, Recurring		Sep. 30, 2013 Total Fair Value, Measurements, Recurring	Dec. 31, 2012 Total Fair Value, Measurements, Recurring	Sep. 30, 2013 Alabama Gas Corporation Fair Value, Measurements, Recurring	Dec. 31, 2012 Alabama Gas Corporation Level 2 Fair Value, Measurements, Recurring	Dec. 31, 2012 Alabama Gas Corporation Level 3 Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current assets			$ (12,712,000)	[1]	$ (3,629,000)			$ 25,005,000	[1]	$ 68,421,000		$ 12,293,000	$ 64,792,000
Noncurrent assets	12,786,000	40,577,000	6,828,000	[1]	18,899,000			5,958,000	[1]	21,678,000		12,786,000	40,577,000
Current liabilities			(40,970,000)	[1]	(2,593,000)			9,042,000	[1]	0		(31,928,000)	(2,593,000)	0	(2,600,000)	0
Noncurrent liabilities	(1,961,000)	(11,305,000)	(2,580,000)		(8,520,000)			1,201,000		(1,080,000)		(1,379,000)	(9,600,000)
Net derivative asset (liability)			(49,434,000)	[1]	4,157,000	[1]		41,206,000	[1]	89,019,000	[1]	(8,228,000)	93,176,000
Impact on fair value of a 10 percent increase or decrease in commodity prices							22,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices							$ 22,000,000

[1] Amounts classified in accordance with accounting guidance which permits offsetting fair value amounts recognized for multiple derivative instruments executed with the same counterparty under a master netting arrangement.