Derivative Commodity Instruments (Details) - Derivatives (USD $)	Sep. 30, 2011		Dec. 31, 2010
Derivative [Line Items]
Deferred tax assets (liabilities), net	$ (43,700,000)		$ 26,800,000
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Noncurrent assets	101,175,000		0
Noncurrent liabilities	(13,331,000)		(112,936,000)
Alabama Gas Corporation | Cash Flow Hedges | 2011 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3.9
Alabama Gas Corporation | Cash Flow Hedges | 2012 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	17.2
Alabama Gas Corporation | Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	1.5
Cash Flow Hedges | Macquarie Bank Limited [Member]
Derivative [Line Items]
Derivative, Net Gain Position	35,800,000
Cash Flow Hedges | J Aron and Company [Member]
Derivative [Line Items]
Derivative, Net Gain Position	29,700,000
Cash Flow Hedges | Shell Energy North America US, L.P. [Member]
Derivative [Line Items]
Derivative, Net Gain Position	23,500,000
Cash Flow Hedges | Citibank, N.A. [Member]
Derivative [Line Items]
Derivative, Net Gain Position	23,000,000
Cash Flow Hedges | J.P. Morgan Ventures Energy Corporation [Member]
Derivative [Line Items]
Derivative, Net Loss Position	200,000
Cash Flow Hedges | 2011 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3.5
Average Contract Price, Per Mcf	6.47
Cash Flow Hedges | 2011 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	9.7
Average Contract Price, Per Mcf	5.63
Cash Flow Hedges | 2011 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	1,334
Average Contract Price, Per Gal	83.73
Cash Flow Hedges | 2011 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	854
Average Contract Price, Per Gal		[1]
Cash Flow Hedges | 2011 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	10.5
Average Contract Price, Per Bbl	0.90
Cash Flow Hedges | 2012 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	11.0
Average Contract Price, Per Mcf	5.07
Cash Flow Hedges | 2012 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	29.5
Average Contract Price, Per Mcf	4.60
Cash Flow Hedges | 2012 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	6,282
Average Contract Price, Per Gal	87.40
Cash Flow Hedges | 2012 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	3,124
Average Contract Price, Per Gal		[1]
Cash Flow Hedges | 2012 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	43.4
Average Contract Price, Per Bbl	0.88
Cash Flow Hedges | 2013 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	8.8
Average Contract Price, Per Mcf	5.30
Cash Flow Hedges | 2013 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	25.1
Average Contract Price, Per Mcf	4.88
Cash Flow Hedges | 2013 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	6,827
Average Contract Price, Per Gal	89.31
Cash Flow Hedges | 2013 | Crude Oil | Basis Swap
Derivative [Line Items]
Total hedged Volumes	2,768
Average Contract Price, Per Gal		[1]
Cash Flow Hedges | 2013 | Natural Gas Liquids | Liquids Swaps
Derivative [Line Items]
Total hedged Volumes	44.5
Average Contract Price, Per Bbl	1.02
Cash Flow Hedges | 2014 | Natural Gas | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	3.0
Average Contract Price, Per Mcf	5.72
Cash Flow Hedges | 2014 | Natural Gas | Basin Specific Swaps
Derivative [Line Items]
Total hedged Volumes	16.8
Average Contract Price, Per Mcf	5.16
Cash Flow Hedges | 2014 | Crude Oil | NYMEX Swaps
Derivative [Line Items]
Total hedged Volumes	5,612
Average Contract Price, Per Gal	90.56
Fair Value Hedging | Expires during 2011 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	3
Fair Value Hedging | Expires during 2012 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	1,735
Fair Value Hedging | Expires during 2013 | Natural Gas
Derivative [Line Items]
Total hedged Volumes	0
Fair Value Hedging | Expires during 2013 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	3,108
Fair Value Hedging | Expires during 2014 | Crude Oil
Derivative [Line Items]
Total hedged Volumes	2,400
Alabama Gas Corporation
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Noncurrent liabilities	(13,050,000)		(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	(44,100,000)		(27,900,000)
Noncurrent liabilities	(13,100,000)		(32,500,000)
Level 2 | Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current assets	51,263,000	[2]	10,316,000	[2]
Noncurrent assets	84,700,000	[2]
Current liabilities	(42,806,000)	[2]	(76,527,000)	[2]
Noncurrent liabilities	(13,050,000)	[2]	(107,452,000)	[2]
Net derivative asset (liability)	80,107,000	[2]	(173,663,000)	[2]
Level 3 | Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current assets	22,680,000	[2]	50,236,000	[2]
Noncurrent assets	16,475,000	[2]
Current liabilities	(2,670,000)	[2]	(1,997,000)	[2]
Noncurrent liabilities	(281,000)	[2]	(5,484,000)	[2]
Net derivative asset (liability)	36,204,000	[2]	42,755,000	[2]
Fair Value, Measurements, Recurring
Derivative Assets and Liabilities, Measured at Fair Value, on Recurring Basis
Current assets	73,943,000		60,552,000
Noncurrent assets	101,175,000
Current liabilities	(45,476,000)		(78,524,000)
Noncurrent liabilities	(13,331,000)		(112,936,000)
Net derivative asset (liability)	$ 116,311,000		$ (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.