Derivative Commodity Instruments - Derivatives Measured at Fair Value on a Recurring Basis (Details) (USD $)									3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Fair Value, Measurements, Recurring [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member]	Mar. 31, 2012 Level 2 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2011 Level 2 Fair Value, Measurements, Recurring [Member]		Mar. 31, 2012 Level 3	Mar. 31, 2012 Level 3 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2011 Level 3 Fair Value, Measurements, Recurring [Member]		Mar. 31, 2012 Alabama Gas Corporation [Member]	Dec. 31, 2011 Alabama Gas Corporation [Member]	Mar. 31, 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			$ 21,679,000	$ 21,792,000	$ (33,849,000)	[1]	$ (14,843,000)	[1]		$ 55,528,000	[1]	$ 36,635,000	[1]
Noncurrent assets	10,608,000	31,056,000	10,608,000	31,056,000	(40,635,000)	[1]	(8,382,000)	[1]		51,243,000	[1]	39,438,000	[1]
Current liabilities			(113,337,000)	(107,290,000)	(111,489,000)	[1]	(98,468,000)	[1]		(1,848,000)	[1]	(8,822,000)	[1]			(47,900,000)	(56,800,000)
Noncurrent liabilities	(61,066,000)	(34,663,000)	(60,713,000)	(34,378,000)	(60,713,000)	[1]	(32,928,000)	[1]		0	[1]	(1,450,000)	[1]	0	(3,070,000)		(3,100,000)
Net derivative asset (liability)			(141,763,000)	(88,820,000)	(246,686,000)	[1]	(154,621,000)	[1]		104,923,000	[1]	65,801,000	[1]
Impact on fair value of a 10 percent increase or decrease in commodity prices									29,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices									$ 400,000

[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.