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Summary of Significant Accounting Policies - Purchase of the Plant Daniel Combined Cycle Generating Units (Details) (Mississippi Power [Member], Plant Daniel Units 3 and 4 [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Mississippi Power [Member] | Plant Daniel Units 3 and 4 [Member]
  
Significant Acquisitions and Disposals [Line Items]  
Fair value adjustment at date of purchase $ 76.1so_SignificantAcquisitionsAndDisposalsAcquisitionCostsAssumptionOfDebtFairValueAdjustment
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Total debt $ 346.1so_SignificantAcquisitionsAndDisposalsAcquisitionCostsAssumptionOfDebtAtFairValue
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