Financial Instruments and Risk Management (Derivative Instruments, Fair Value) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Derivative [Line Items]
Noncurrent assets	$ 31,056		$ 0
Noncurrent liabilities	(34,663)		(112,936)
Fair Value, Measurements, Recurring
Derivative [Line Items]
Current assets	21,792		60,552
Noncurrent assets	31,056
Current liabilities	(107,290)		(78,524)
Noncurrent liabilities	(34,378)		(112,936)
Net derivative asset (liability)	(88,820)		(130,908)
Level 2 | Fair Value, Measurements, Recurring
Derivative [Line Items]
Current assets	(14,843)	[1]	10,316	[1]
Noncurrent assets	(8,382)	[1]
Current liabilities	(98,468)	[1]	(76,527)	[1]
Noncurrent liabilities	(32,928)	[1]	(107,452)	[1]
Net derivative asset (liability)	(154,621)	[1]	(173,663)	[1]
Level 3 | Fair Value, Measurements, Recurring
Derivative [Line Items]
Current assets	36,635	[1]	50,236	[1]
Noncurrent assets	39,438	[1]
Current liabilities	(8,822)	[1]	(1,997)	[1]
Noncurrent liabilities	(1,450)	[1]	(5,484)	[1]
Net derivative asset (liability)	65,801	[1]	42,755	[1]
Alabama Gas Corporation
Derivative [Line Items]
Noncurrent liabilities	(3,070)		(32,461)
Alabama Gas Corporation | Level 2 | Fair Value, Measurements, Recurring
Derivative [Line Items]
Current liabilities	(56,800)		(27,900)
Noncurrent liabilities	$ (3,100)		$ (32,500)

[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.