Derivative Commodity Instruments - Derivatives Measured at Fair Value on a Recurring Basis (Details) (USD $)								3 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2012 Fair Value, Measurements, Recurring	Jun. 30, 2013 Level 2 Fair Value, Measurements, Recurring		Dec. 31, 2012 Level 2 Fair Value, Measurements, Recurring		Jun. 30, 2013 Level 3	Jun. 30, 2013 Level 3 Fair Value, Measurements, Recurring		Dec. 31, 2012 Level 3 Fair Value, Measurements, Recurring		Jun. 30, 2013 Total Fair Value, Measurements, Recurring	Jun. 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			$ 64,792,000	$ 8,108,000	[1]	$ (3,629,000)			$ 37,973,000	[1]	$ 68,421,000		$ 46,081,000
Noncurrent assets	39,006,000	40,577,000	40,577,000	25,977,000	[1]	18,899,000			13,029,000	[1]	21,678,000		39,006,000
Current liabilities			(2,593,000)	(12,635,000)	[1]	(2,593,000)			129,000	[1]	0		(12,506,000)	0	(2,600,000)	0
Noncurrent liabilities	(470,000)	(11,305,000)	(9,600,000)			(8,520,000)					(1,080,000)
Net derivative asset			93,176,000	21,450,000	[1]	4,157,000	[1]		51,131,000	[1]	89,019,000	[1]	72,581,000
Impact on fair value of a 10 percent increase or decrease in commodity prices								26,000,000
Impact on results of operations of a 10 percent increase or decrease in commodity prices								$ 9,800,000

[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.