v3.19.2
Derivatives - Pre-tax Effects of Derivatives Designated as Cash Flow Hedging Instruments (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]          
Collateral already posted, aggregate fair value $ 178   $ 178    
Gain (Loss) Recognized in OCI on Derivative (44) $ (73) (82) $ (11)  
GEORGIA POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative (37) 0 (37) 0  
SOUTHERN POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative (3) (74) (41) (11)  
SOUTHERN Co GAS          
Derivative Instruments, Gain (Loss) [Line Items]          
Collateral already posted, aggregate fair value 178   178   $ 277
Energy-related derivatives          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (65)   (65)   (40)
Gain (Loss) Recognized in OCI on Derivative (6) 0 (6) 12  
Energy-related derivatives | Other regulatory assets, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (48)   (48)   (19)
Energy-related derivatives | Other regulatory assets, deferred          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (23)   (23)   (16)
Energy-related derivatives | Assets held for sale, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses)         (6)
Energy-related derivatives | Other regulatory liabilities, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 6   6   1
Energy-related derivatives | ALABAMA POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (13)   (13)   (6)
Energy-related derivatives | ALABAMA POWER CO | Other regulatory assets, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (11)   (11)   (3)
Energy-related derivatives | ALABAMA POWER CO | Other regulatory assets, deferred          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (5)   (5)   (3)
Energy-related derivatives | ALABAMA POWER CO | Assets held for sale, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses)         0
Energy-related derivatives | ALABAMA POWER CO | Other regulatory liabilities, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 3   3   0
Energy-related derivatives | GEORGIA POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (37)   (37)   (15)
Energy-related derivatives | GEORGIA POWER CO | Other regulatory assets, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (25)   (25)   (6)
Energy-related derivatives | GEORGIA POWER CO | Other regulatory assets, deferred          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (12)   (12)   (9)
Energy-related derivatives | GEORGIA POWER CO | Assets held for sale, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses)         0
Energy-related derivatives | GEORGIA POWER CO | Other regulatory liabilities, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 0   0   0
Energy-related derivatives | MISSISSIPPI POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (16)   (16)   (6)
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory assets, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (10)   (10)   (2)
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory assets, deferred          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (6)   (6)   (4)
Energy-related derivatives | MISSISSIPPI POWER CO | Assets held for sale, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses)         0
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory liabilities, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 0   0   0
Energy-related derivatives | SOUTHERN POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative (2) (1) (2) 10  
Energy-related derivatives | SOUTHERN Co GAS          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 1   1   (7)
Collateral already posted, aggregate fair value 12   12    
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory assets, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) (2)   (2)   (8)
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory assets, deferred          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 0   0   0
Energy-related derivatives | SOUTHERN Co GAS | Assets held for sale, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses)         0
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory liabilities, current          
Derivative Instruments, Gain (Loss) [Line Items]          
Total energy-related derivative gains (losses) 3   3   $ 1
Interest rate derivatives          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative (37) 0 (37) (2)  
Foreign currency derivatives          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative (1) (73) (39) (21)  
Foreign currency derivatives | SOUTHERN POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative (1) (73) (39) (21)  
Total interest expense, net of amounts capitalized | Interest rate derivatives | GEORGIA POWER CO          
Derivative Instruments, Gain (Loss) [Line Items]          
Gain (Loss) Recognized in OCI on Derivative $ (37) $ 0 $ (37) $ 0