Derivative Commodity Instruments - Derivatives Measured at Fair Value on a Recurring Basis (Details) (USD $)								3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2012 Fair Value, Measurements, Recurring	Mar. 31, 2013 Level 2 Fair Value, Measurements, Recurring		Dec. 31, 2012 Level 2 Fair Value, Measurements, Recurring		Mar. 31, 2013 Level 3	Mar. 31, 2013 Level 3 Fair Value, Measurements, Recurring		Dec. 31, 2012 Level 3 Fair Value, Measurements, Recurring		Mar. 31, 2013 Total Fair Value, Measurements, Recurring	Mar. 31, 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			$ 64,792,000	$ (12,452,000)	[1]	$ (3,629,000)			$ 23,142,000	[1]	$ 68,421,000		$ 10,690,000
Noncurrent assets	26,199,000	40,577,000	40,577,000	12,194,000	[1]	18,899,000			14,005,000	[1]	21,678,000		26,199,000
Current liabilities			(2,593,000)	(24,834,000)	[1]	(2,593,000)			(7,965,000)	[1]	0		(32,799,000)	0	(2,600,000)	0
Noncurrent liabilities	(9,705,000)	(11,305,000)	(9,600,000)	(5,626,000)	[1]	(8,520,000)			(2,723,000)	[1]	(1,080,000)		(8,349,000)
Net derivative asset (liability)			93,176,000	(30,718,000)	[1]	4,157,000	[1]		26,459,000	[1]	89,019,000	[1]	(4,259,000)
Impact on fair value of a 10 percent increase or decrease in commodity prices								28,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices								$ 5,200,000

[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.