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Fair Value Measurements - Narrative (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Fair Value, Option, Quantitative Disclosures [Line Items]  
Increase (decrease) in fair value of funds including reinvested interest and dividends $ 65so_IncreaseDecreaseInFairValueOfFundsIncludingReinvestedInterestAndDividends
Georgia Power [Member]  
Fair Value, Option, Quantitative Disclosures [Line Items]  
Notices Of Withdrawal Foreign Equity Funds 20.00%so_NoticesOfWithdrawalForeignEquityFunds
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Increase (decrease) in fair value of funds including reinvested interest and dividends 18so_IncreaseDecreaseInFairValueOfFundsIncludingReinvestedInterestAndDividends
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
Georgia Power [Member] | Other - commingled funds [Member]  
Fair Value, Option, Quantitative Disclosures [Line Items]  
Maximum Number of Days Related to Dollar Weighted Average Portfolio Maturities Regarding Commingled Funds 90 days
Georgia Power [Member] | Minimum [Member]  
Fair Value, Option, Quantitative Disclosures [Line Items]  
Withdrawal Of Foreign Equity Fund Investment 1so_WithdrawalOfForeignEquityFundInvestment
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Foreign Equity Fund Investment 10so_ForeignEquityFundInvestment
/ dei_LegalEntityAxis
= so_GeorgiaPowerMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Alabama Power [Member]  
Fair Value, Option, Quantitative Disclosures [Line Items]  
Increase (decrease) in fair value of funds including reinvested interest and dividends $ 47so_IncreaseDecreaseInFairValueOfFundsIncludingReinvestedInterestAndDividends
/ dei_LegalEntityAxis
= so_AlabamaPowerMember