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FINANCING - Outstanding Classes of Capital Stock Narrative (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Debt Instrument [Line Items]    
Stock issued employee and director stock plans (in shares) 19,500,000  
Proceeds from issuance of shares under share-based compensation plans | $ $ 844,000,000  
Number of shares reserved for issuance to stock-based compensation plan (in shares) 104,000,000  
Southern Company Common Stock    
Debt Instrument [Line Items]    
Remaining shares available for awards (in shares) 9,000,000  
Alabama Power | 5.00% Class A Preferred Stock    
Debt Instrument [Line Items]    
Shares outstanding (in shares) 10,000,000 10,000,000
Temporary equity, dividend rate percentage 0.05 0.0500
Preferred stock, par value (in dollars per share) | $ / shares   $ 1
Temporary equity, par value (in dollars per share) | $ / shares $ 25 $ 25
Alabama Power | 5.00% Class A Preferred Stock | Aggregate Stated Capital    
Debt Instrument [Line Items]    
Debt instrument, face amount | $   $ 250,000,000
Alabama Power | 5.30% Class A Preferred Stock    
Debt Instrument [Line Items]    
Shares outstanding (in shares)   2,000,000
Temporary equity, dividend rate percentage   0.0650
Alabama Power | 5.30% Class A Preferred Stock | Aggregate Stated Capital    
Debt Instrument [Line Items]    
Debt instrument, face amount | $   $ 50,000,000
Alabama Power | 5.625% Preference Stock    
Debt Instrument [Line Items]    
Shares outstanding (in shares)   6,000,000
Temporary equity, dividend rate percentage   0.0645
Alabama Power | 5.625% Preference Stock | Aggregate Stated Capital    
Debt Instrument [Line Items]    
Debt instrument, face amount | $   $ 150,000,000
Alabama Power | 5.83% Class A Preferred Stock    
Debt Instrument [Line Items]    
Shares outstanding (in shares)   1,520,000
Temporary equity, dividend rate percentage   0.0583
Alabama Power | 5.83% Class A Preferred Stock | Aggregate Stated Capital    
Debt Instrument [Line Items]    
Debt instrument, face amount | $   $ 38,000,000