Derivative Commodity Instruments (Details) - Derivatives (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010	Dec. 31, 2010
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ 49,449,000		$ 42,917,000	$ (47,425,000)		$ 98,385,000
Deferred Tax Assets, Derivative Instruments	55,600,000			55,600,000			26,800,000
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Noncurrent assets	1,002,000			1,002,000			0
Noncurrent liabilities	(128,505,000)			(128,505,000)			(112,936,000)
Alabama Gas Corporation | Cash Flow Hedges | 2011 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	6.0			6.0
Alabama Gas Corporation | Cash Flow Hedges | 2012 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	17.2			17.2
Alabama Gas Corporation | Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	1.5			1.5
Cash Flow Hedges | Morgan Stanley Capital Group, Inc.
Derivative [Line Items]
Derivative, Net Loss Position	57,900,000			57,900,000
Cash Flow Hedges | Citibank, N.A. [Member]
Derivative [Line Items]
Derivative, Net Loss Position	36,200,000			36,200,000
Cash Flow Hedges | J Aron and Company [Member]
Derivative [Line Items]
Derivative, Net Loss Position	20,800,000			20,800,000
Cash Flow Hedges | Shell Energy North America US, L.P. [Member]
Derivative [Line Items]
Derivative, Net Loss Position	20,200,000			20,200,000
Cash Flow Hedges | Barclays Bank PLC [Member]
Derivative [Line Items]
Derivative, Net Gain Position	6,700,000			6,700,000
Cash Flow Hedges | 2011 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	7.1			7.1
Average Contract Price, Per Mcf	6.47			6.47
Cash Flow Hedges | 2011 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	19.4			19.4
Average Contract Price, Per Mcf	5.65			5.65
Cash Flow Hedges | 2011 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	2,242			2,242
Average Contract Price, Per Gal	80.47			80.47
Cash Flow Hedges | 2011 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	1,103			1,103
Average Contract Price, Per Gal		[1]			[1]
Cash Flow Hedges | 2011 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	21.1			21.1
Average Contract Price, Per Bbl	0.90			0.90
Cash Flow Hedges | 2012 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	11.0			11.0
Average Contract Price, Per Mcf	5.07			5.07
Cash Flow Hedges | 2012 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	29.5			29.5
Average Contract Price, Per Mcf	4.60			4.60
Cash Flow Hedges | 2012 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3,881			3,881
Average Contract Price, Per Gal	83.53			83.53
Cash Flow Hedges | 2012 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	753			753
Average Contract Price, Per Gal		[1]			[1]
Cash Flow Hedges | 2012 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	39.9			39.9
Average Contract Price, Per Bbl	0.86			0.86
Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	8.8			8.8
Average Contract Price, Per Mcf	5.30			5.30
Cash Flow Hedges | 2013 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	25.1			25.1
Average Contract Price, Per Mcf	4.88			4.88
Cash Flow Hedges | 2013 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3,296			3,296
Average Contract Price, Per Gal	85.97			85.97
Cash Flow Hedges | 2013 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	35.2			35.2
Average Contract Price, Per Bbl	1.02			1.02
Cash Flow Hedges | 2014 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3.0			3.0
Average Contract Price, Per Mcf	5.72			5.72
Cash Flow Hedges | 2014 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	6.0			6.0
Average Contract Price, Per Mcf	5.34			5.34
Cash Flow Hedges | 2014 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	2,818			2,818
Average Contract Price, Per Gal	87.92			87.92
Fair Value Hedging | Crude Oil
Derivative [Line Items]
Total hedged Volumes	7,000			7,000
Alabama Gas Corporation
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Noncurrent liabilities	(17,657,000)			(17,657,000)			(32,461,000)
Alabama Gas Corporation | Level 2 | Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current liabilities	(32,200,000)			(32,200,000)			(27,900,000)
Noncurrent liabilities	(17,700,000)			(17,700,000)			(32,500,000)
Level 2 | Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current assets	(7,773,000)	[2]		(7,773,000)	[2]		10,316,000	[2]
Noncurrent assets	78,000	[2]		78,000	[2]
Current liabilities	(81,752,000)	[2]		(81,752,000)	[2]		(76,527,000)	[2]
Noncurrent liabilities	(126,650,000)	[2]		(126,650,000)	[2]		(107,452,000)	[2]
Net derivative asset (liability)	(216,097,000)	[2]		(216,097,000)	[2]		(173,663,000)	[2]
Level 3 | Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current assets	19,778,000	[2]		19,778,000	[2]		50,236,000	[2]
Noncurrent assets	924,000	[2]		924,000	[2]
Current liabilities	(5,401,000)	[2]		(5,401,000)	[2]		(1,997,000)	[2]
Noncurrent liabilities	(1,855,000)	[2]		(1,855,000)	[2]		(5,484,000)	[2]
Net derivative asset (liability)	13,446,000	[2]		13,446,000	[2]		42,755,000	[2]
Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current assets	12,005,000			12,005,000			60,552,000
Noncurrent assets	1,002,000			1,002,000
Current liabilities	(87,153,000)			(87,153,000)			(78,524,000)
Noncurrent liabilities	(128,505,000)			(128,505,000)			(112,936,000)
Net derivative asset (liability)	$ (202,651,000)			$ (202,651,000)			$ (130,908,000)

[1] Average contract prices are not meaningful due to the varying nature of each contract.

[2] 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.