ACQUISITIONS AND DISPOSITIONS - Schedule of Construction Projects (Details) - SOUTHERN POWER CO - MW	1 Months Ended	6 Months Ended
	May 31, 2020	Dec. 31, 2020	Mar. 31, 2020	Oct. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)			300
Reading | Reading
Business Acquisition [Line Items]
Ownership percentage by parent					100.00%
Reading | Reading | Class B Membership Interest
Business Acquisition [Line Items]
Ownership percentage by parent					100.00%
Skookumchuck | Skookumchuck
Business Acquisition [Line Items]
Ownership percentage by parent				100.00%
Scenario, Forecast | Reading
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	200
PPA Contract Period	12 years
Scenario, Forecast | Skookumchuck
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)		136
PPA Contract Period		20 years