SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Concentration of Revenue (Details) - Southern Power - Sales Revenue, Goods, Net - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Georgia Power
Concentration Risk [Line Items]
Percentage of total revenues for top three customers	9.00%	9.80%	11.30%
Duke Energy Corporation
Concentration Risk [Line Items]
Percentage of total revenues for top three customers		6.80%	6.70%
Southern California Edison
Concentration Risk [Line Items]
Percentage of total revenues for top three customers	6.80%	6.20%
Morgan Stanley Capital Group
Concentration Risk [Line Items]
Percentage of total revenues for top three customers	4.90%		4.50%