ACQUISITIONS AND DISPOSITIONS - Construction Projects (Details) - Southern Power - MW	1 Months Ended		3 Months Ended
	Dec. 31, 2019	May 31, 2019	Jun. 30, 2020	Oct. 31, 2019	Aug. 31, 2018	May 31, 2018
Mankato
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	385	385
PPA Contract Period		20 years
Wild Horse Mountain
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	100
PPA Contract Period	20 years
Wild Horse Mountain | Wild Horse Mountain
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent						100.00%
Wild Horse Mountain | Class B Membership Interest | Wild Horse Mountain
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent	100.00%
Reading | Reading
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent					100.00%
Reading | Class B Membership Interest | Reading
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent					100.00%
Skookumchuck | Skookumchuck
Business Acquisition [Line Items]
Noncontrolling ownership percentage held 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