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ACQUISITIONS AND DISPOSITIONS - Schedule of Acquisition (Details) - SOUTHERN POWER CO - MW
 1 Months Ended
May 31, 2020
Jun. 30, 2020
Business Acquisition [Line Items]    
Approximate nameplate capacity (in MWs)   300
Beech Ridge II    
Business Acquisition [Line Items]    
Approximate nameplate capacity (in MWs) 56  
PPA Contract Period 12 years  
Beech Ridge II | Beech Ridge II    
Business Acquisition [Line Items]    
Ownership percentage by parent 100.00%