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Segment and Related Information - Financial Data for Products and Services (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Revenue from External Customer [Line Items]    
Electric Utilities' Revenues $ 4,172us-gaap_ElectricUtilityRevenue $ 4,627us-gaap_ElectricUtilityRevenue
Retail [Member]    
Revenue from External Customer [Line Items]    
Electric Utilities' Revenues 3,542us-gaap_ElectricUtilityRevenue
/ us-gaap_ProductOrServiceAxis
= so_RetailMember		 3,858us-gaap_ElectricUtilityRevenue
/ us-gaap_ProductOrServiceAxis
= so_RetailMember
Wholesale [Member]    
Revenue from External Customer [Line Items]    
Electric Utilities' Revenues 467us-gaap_ElectricUtilityRevenue
/ us-gaap_ProductOrServiceAxis
= so_WholesaleMember		 604us-gaap_ElectricUtilityRevenue
/ us-gaap_ProductOrServiceAxis
= so_WholesaleMember
Other [Member]    
Revenue from External Customer [Line Items]    
Electric Utilities' Revenues $ 163us-gaap_ElectricUtilityRevenue
/ us-gaap_ProductOrServiceAxis
= so_OtherElectricRevenueMember		 $ 165us-gaap_ElectricUtilityRevenue
/ us-gaap_ProductOrServiceAxis
= so_OtherElectricRevenueMember