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Segment and Related Information - Financial Data for Business Segments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Segment Reporting Information [Line Items]        
Number of States in which Entity Operates 4us-gaap_NumberOfStatesInWhichEntityOperates      
Operating revenues $ 4,183us-gaap_Revenues $ 4,644us-gaap_Revenues    
Segment net income (loss) 508us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic [1],[2] 351us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic [1],[2]    
Total assets 71,610us-gaap_Assets   70,923us-gaap_Assets  
Kemper Igcc [Member]        
Segment Reporting Information [Line Items]        
Pre-tax charge to income 9so_PreTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember		 380so_PreTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember		 868so_PreTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember		 1,200so_PreTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember
After tax charge to income 6so_AfterTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember		 235so_AfterTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember		 536so_AfterTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember		 729so_AfterTaxChargeToIncome
/ us-gaap_PublicUtilityAxis
= so_KemperIgccMember
Electric Utilities [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 4,172us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember		 4,627us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember		    
Segment net income (loss) 510us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember		 [1],[2] 351us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember		 [1],[2]    
Total assets 70,821us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember		   70,063us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember		  
Traditional Operating Companies [Member] | Electric Utilities [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 3,948us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_TraditionalOperatingCompaniesMember		 4,378us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_TraditionalOperatingCompaniesMember		    
Segment net income (loss) 477us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_TraditionalOperatingCompaniesMember		 [1],[2] 318us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_TraditionalOperatingCompaniesMember		 [1],[2]    
Total assets 65,530us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_TraditionalOperatingCompaniesMember		   64,644us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_TraditionalOperatingCompaniesMember		  
Southern Power [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 114us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		 72us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		    
Southern Power [Member] | Electric Utilities [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 348us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		 351us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		    
Segment net income (loss) 33us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		 [1],[2] 33us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		 [1],[2]    
Total assets 5,564us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		   5,550us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= so_SouthernPowerMember		  
Eliminations [Member]        
Segment Reporting Information [Line Items]        
Operating revenues (29)us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		 (24)us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		    
Segment net income (loss) (5)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		 [1],[2] 0us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		 [1],[2]    
Total assets (302)us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		   (296)us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		  
Eliminations [Member] | Electric Utilities [Member]        
Segment Reporting Information [Line Items]        
Operating revenues (124)us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		 (102)us-gaap_Revenues
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		    
Segment net income (loss) 0us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		 [1],[2] 0us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		 [1],[2]    
Total assets (273)us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		   (131)us-gaap_Assets
/ us-gaap_ProductOrServiceAxis
= so_ElectricUtilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_IntersegmentEliminationMember		  
All Other [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 40us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		 41us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		    
Segment net income (loss) 3us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		 [1],[2] 0us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		 [1],[2]    
Total assets $ 1,091us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		   $ 1,156us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember		  
[1] Segment net income (loss) for the traditional operating companies for the three months ended March 31, 2015 and March 31, 2014 includes a $9 million pre-tax charge ($6 million after tax) and a $380 million pre-tax charge ($235 million after tax), respectively, for estimated probable losses on the Kemper IGCC. See Note (B) under "Integrated Coal Gasification Combined Cycle – Kemper IGCC Schedule and Cost Estimate" herein for additional information.
[2] After dividends on preferred and preference stock of subsidiaries.