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Fair Value Measurements - Fair Value Measurements Calculated at Net Asset Value Per Share (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Foreign equity funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value $ 126,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_ForeignEquityMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_ForeignEquityMember
Redemption Frequency Monthly
Redemption Notice Period 5 days
Redemption Notice Period, Description 5 days
Equity - commingled funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 65,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_EquitycommingledfundsMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_EquitycommingledfundsMember
Redemption Frequency Daily/Monthly
Redemption Notice Period 7 days
Redemption Notice Period, Description Daily/7 days
Debt - Commingled Funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 16,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_DebtCommingledFundsMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_DebtCommingledFundsMember
Redemption Frequency Daily
Redemption Notice Period 5 days
Redemption Notice Period, Description 5 days
Other - commingled funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 6,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_OtherCommingledFundsMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_OtherCommingledFundsMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Other - money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 11,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_OthermoneymarketfundsMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_OthermoneymarketfundsMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Trust owned life insurance [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 118,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_TrustOwnedLifeInsuranceMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_TrustOwnedLifeInsuranceMember
Redemption Frequency Daily
Redemption Notice Period 15 days
Redemption Notice Period, Description 15 days
Money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 863,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Alabama Power [Member] | Equity - commingled funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 65,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_EquitycommingledfundsMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_EquitycommingledfundsMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Redemption Frequency Daily/Monthly
Redemption Notice Period 7 days
Redemption Notice Period, Description Daily/7 days
Alabama Power [Member] | Debt - Commingled Funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 16,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_DebtCommingledFundsMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_DebtCommingledFundsMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Redemption Frequency Daily
Redemption Notice Period 5 days
Redemption Notice Period, Description 5 days
Alabama Power [Member] | Trust owned life insurance [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 118,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_TrustOwnedLifeInsuranceMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_TrustOwnedLifeInsuranceMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Redemption Frequency Daily
Redemption Notice Period 15 days
Redemption Notice Period, Description 15 days
Alabama Power [Member] | Money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 287,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_AlabamaPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Georgia Power [Member] | Foreign equity funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 126,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_ForeignEquityMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_ForeignEquityMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Redemption Frequency Monthly
Redemption Notice Period 5 days
Redemption Notice Period, Description 5 days
Georgia Power [Member] | Other - commingled funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 6,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_OtherCommingledFundsMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_OtherCommingledFundsMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Georgia Power [Member] | Other - money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 11,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= so_OthermoneymarketfundsMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= so_OthermoneymarketfundsMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Georgia Power [Member] | Money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 406,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Gulf Power [Member] | Money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 18,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_GulfPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_GulfPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Mississippi Power [Member] | Money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 110,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_MississippiPowerMember
Unfunded Commitments 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_MississippiPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable
Southern Power [Member] | Money market funds [Member]  
Fair value measurements of investments calculated at net asset value per share as well as the nature and risk of those investments  
Fair Value 3,000,000us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_SouthernPowerMember
Unfunded Commitments $ 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MoneyMarketFundsMember
/ dei_LegalEntityAxis
= so_SouthernPowerMember
Redemption Frequency Daily
Redemption Notice Period, Description Not applicable