Derivative Commodity Instruments - Derivatives Measured at Fair Value on a Recurring Basis (Details) (USD $)									3 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member]	Sep. 30, 2012 Level 2 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2011 Level 2 Fair Value, Measurements, Recurring [Member]		Sep. 30, 2012 Level 3	Sep. 30, 2012 Level 3 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2011 Level 3 Fair Value, Measurements, Recurring [Member]		Sep. 30, 2012 Alabama Gas Corporation [Member]	Dec. 31, 2011 Alabama Gas Corporation [Member]	Sep. 30, 2012 Alabama Gas Corporation [Member] Level 2 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2011 Alabama Gas Corporation [Member] Level 2 Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current assets			$ 36,275,000	$ 21,792,000	$ (1,048,000)	[1]	$ (14,843,000)			$ 37,323,000	[1]	$ 36,635,000
Noncurrent assets	43,137,000	31,056,000	43,137,000	31,056,000	18,133,000	[1]	(8,382,000)			25,004,000	[1]	39,438,000
Current liabilities			(31,937,000)	(107,290,000)	(30,588,000)	[1]	(98,468,000)			(1,349,000)	[1]	(8,822,000)				(15,000,000)	(56,800,000)
Noncurrent liabilities	(11,110,000)	(34,663,000)	(9,055,000)	(34,378,000)	(5,358,000)	[1]	(32,928,000)			(3,697,000)	[1]	(1,450,000)		0	(3,070,000)		(3,100,000)
Net derivative asset (liability)			38,420,000	(88,820,000)	(18,861,000)	[1]	(154,621,000)	[1]		57,281,000	[1]	65,801,000	[1]
Impact on fair value of a 10 percent increase or decrease in commodity prices									35,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices									$ 5,900,000

[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.