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Regulatory Matters - MPC (Details) - MISSISSIPPI POWER CO
$ in Millions
Jun. 08, 2021
USD ($)
Apr. 06, 2021
USD ($)
Jun. 30, 2021
USD ($)
Apr. 15, 2021
MW
Dec. 31, 2020
MW
Loss Contingencies [Line Items]          
Approved rate increase | $ $ 16 $ 28      
Approved rate increase, percentage 1.80%        
Required fossil-stream generation retirements (in MWs) | MW         950
Public utilities, valorem taxes previously recovered, amount | $   $ 19      
Environmental Compliance Overview Plan          
Loss Contingencies [Line Items]          
Approved rate increase | $ $ 9        
Plant Watson Unit 4          
Loss Contingencies [Line Items]          
Required fossil-stream generation retirements (in MWs) | MW       268  
Plant Greene County Units 1 and 2          
Loss Contingencies [Line Items]          
Required fossil-stream generation retirements (in MWs) | MW       103  
Plant Greene County Units 1 and 2 | ALABAMA POWER CO          
Loss Contingencies [Line Items]          
Proportionate ownership share       40.00%  
Plant Daniel Units 1 and 2          
Loss Contingencies [Line Items]          
Required fossil-stream generation retirements (in MWs) | MW       502  
Proportionate ownership share       50.00%  
Remaining net book value | $     $ 522