Regulatory Matters (Details) (USD $)	1 Months Ended		3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2010	Jun. 30, 2011	Sep. 30, 2010	Dec. 31, 1998
Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent	$ 105,365,000		$ 109,244,000	$ 109,244,000
Alabama Gas Corporation | Enhanced stability reserve
Regulatory Matters [Line Items]
Regulatory Assets, Noncurrent	3,794,000		0	0
Alabama Gas Corporation
Regulatory Matters [Line Items]
Enahanced Stability Reserve Extension Period				7 years
Revenue Impact of Rate Adjustments	(1,300,000)	10,200,000	(1,200,000)	(6,700,000)	(10,600,000)
Adjustment Requirement Threshold for Return on Equity as Percentage of Operations and Maintenance Expenses to Consumer Price Index				0.75
Return on Equity, Adjustment Requirements, Description				RSE limits the utility’s equity upon which a return is permitted to 55 percent of total capitalization, subject to certain adjustments. Under the inflation-based Cost Control Measurement (CCM) established by the APSC, if the percentage change in operations and maintenance (O&M) expense on an aggregate basis falls within a range of 0.75 points above or below the percentage change in the Consumer Price Index For All Urban Consumers (Index Range), no adjustment is required. If the change in O&M expense on an aggregate basis exceeds the Index Range, three-quarters of the difference is returned to customers. To the extent the change is less than the Index Range, the utility benefits by one-half of the difference through future rate adjustments. The O&M expense base for measurement purposes will be set at the prior year’s actual O&M expense amount unless the Company exceeds the top of the Index Range in two successive years, in which case the base for the following year will be set at the top of the Index Range.
Extraordinary Bad Debt Expense, Excluded from CCM Calculation							2,500,000
Negative Revenue Variance, Large Commercial and Industrial Customer Budget, Minimum Amount, Charged to Enhanced Stability Reserve								250,000
Accretion to Enhanced Stability Reserve, Monthly								40,000
Amortization Expense, Enhanced Stability Reserve, Annual Amount				660,000
Enahanced Stability Reserve Amortization Period						5 years
Enhanced Stability Reserve Refund Period						9 years
Regulatory Assets, Noncurrent	105,365,000		109,244,000	109,244,000
Alabama Gas Corporation | APSC Approved Expension of ESR, November 1, 2010
Regulatory Matters [Line Items]
Negative Revenue Variance, Large Commercial and Industrial Customer Budget, Minimum Amount, Charged to Enhanced Stability Reserve				350,000
Accretion to Enhanced Stability Reserve, Monthly				55,000
Alabama Gas Corporation | APSC Approved Expension of ESR, November 1, 2010 | Force Majeure Event Costs
Regulatory Matters [Line Items]
Extraordinary Operating and Maintenance Expenses, Minimum Amount, Charged to Enhanced Stability Reserve				275,000
Alabama Gas Corporation | APSC Approved Expension of ESR, November 1, 2010 | Force Majeure Events Costs
Regulatory Matters [Line Items]
Extraordinary Operating and Maintenance Expenses, Minimum Amount, Charged to Enhanced Stability Reserve				412,500
Alabama Gas Corporation | APSC Approved Expension of ESR, November 1, 2010 | Environmental Response and Self Insurance Costs
Regulatory Matters [Line Items]
Extraordinary Operating and Maintenance Expenses, Minimum Amount, Charged to Enhanced Stability Reserve				1,000,000
Alabama Gas Corporation | Force Majeure Event Costs
Regulatory Matters [Line Items]
Extraordinary Operating and Maintenance Expenses, Minimum Amount, Charged to Enhanced Stability Reserve								200,000
Alabama Gas Corporation | Minimum
Regulatory Matters [Line Items]
Allowed Return on Average Common Equity				13.15%
Alabama Gas Corporation | Maximum
Regulatory Matters [Line Items]
Allowed Return on Average Common Equity				13.65%
Rate Increases as Percentage of Prior Year Revenues				4.00%
Limitation on Return on Equity as Percentage of Total Capitalization				55.00%
Enhanced Stability Reserve								$ 4,000,000