Financial Instruments and Risk Management (Derivative Instruments, Fair Value) (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Noncurrent assets	$ 40,577,000		$ 31,056,000
Noncurrent liabilities	(11,305,000)		(34,663,000)
Expected change in fair value of open Level 3 derviated contracts	27,000,000
Resulting impact upon the results of operations associated with open Level 3 mark-to-market derivative contracts	2,500,000
Fair Value, Measurements, Recurring
Derivative [Line Items]
Current assets	64,792,000		21,792,000
Noncurrent assets	40,577,000		31,056,000
Current liabilities	(2,593,000)		(107,290,000)
Noncurrent liabilities	(9,600,000)		(34,378,000)
Net derivative asset (liability)	93,176,000		(88,820,000)
Level 2 | Fair Value, Measurements, Recurring
Derivative [Line Items]
Current assets	(3,629,000)	[1]	(14,843,000)	[1]
Noncurrent assets	18,899,000	[1]	(8,382,000)	[1]
Current liabilities	(2,593,000)	[1]	(98,468,000)	[1]
Noncurrent liabilities	(8,520,000)	[1]	(32,928,000)	[1]
Net derivative asset (liability)	4,157,000	[1]	(154,621,000)	[1]
Level 3 | Fair Value, Measurements, Recurring
Derivative [Line Items]
Current assets	68,421,000	[1]	36,635,000	[1]
Noncurrent assets	21,678,000	[1]	39,438,000	[1]
Current liabilities	0	[1]	(8,822,000)	[1]
Noncurrent liabilities	(1,080,000)	[1]	(1,450,000)	[1]
Net derivative asset (liability)	89,019,000	[1]	65,801,000	[1]
Alabama Gas Corporation
Derivative [Line Items]
Noncurrent liabilities	0		(3,070,000)
Alabama Gas Corporation | Level 2 | Fair Value, Measurements, Recurring
Derivative [Line Items]
Current liabilities	(2,600,000)		(56,800,000)
Noncurrent liabilities			$ (3,100,000)

[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.