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REGULATORY MATTERS - MPC Integrated Resource Plan (Details)
$ in Millions
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Sep. 30, 2021
MW
Dec. 31, 2019
MW
Public Utilities, General Disclosures [Line Items]        
Net regulatory assets | $ $ 5,833 $ 4,711    
Mississippi Power        
Public Utilities, General Disclosures [Line Items]        
Required fossil-stream generation retirements (in MWs) | MW       950
Net regulatory assets | $ 4 $ 46    
Mississippi Power | Plant Watson Unit 4        
Public Utilities, General Disclosures [Line Items]        
Required fossil-stream generation retirements (in MWs) | MW     268  
Mississippi Power | Plant Greene County Units 1 and 2        
Public Utilities, General Disclosures [Line Items]        
Required fossil-stream generation retirements (in MWs) | MW     103  
Percent ownership     40.00%  
Mississippi Power | Plant Greene County Units 1 and 2 | Alabama Power        
Public Utilities, General Disclosures [Line Items]        
Percent ownership     40.00%  
Mississippi Power | Plant Daniel        
Public Utilities, General Disclosures [Line Items]        
Required fossil-stream generation retirements (in MWs) | MW     502  
Percent ownership     50.00%  
Net regulatory assets | $ 499      
Estimated amount remaining at retirement | $ $ 397