ACQUISITIONS AND DISPOSITIONS - Schedule of Construction Projects (Details) $ in Millions	1 Months Ended			3 Months Ended
	Nov. 30, 2020 MW	Jun. 30, 2020 USD ($) MW	May 31, 2020 MW	Jun. 30, 2021 MW	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Oct. 31, 2019	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Operating lease ROU assets, net | $					$ 1,791	$ 1,800
SOUTHERN POWER CO
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW) | MW		300
Operating lease ROU assets, net | $					$ 395	$ 369
SOUTHERN POWER CO | Reading
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW) | MW			200
PPA Contract Period			12 years
SOUTHERN POWER CO | Reading | Reading
Business Acquisition [Line Items]
Ownership percentage by parent								100.00%
Operating lease ROU assets, net | $								$ 24
Operating lease obligations | $								$ 24
Proceeds from tax equity transaction | $		$ 156
SOUTHERN POWER CO | Reading | Reading | Class B Membership Interest
Business Acquisition [Line Items]
Ownership percentage by parent					100.00%
SOUTHERN POWER CO | Skookumchuck | Skookumchuck
Business Acquisition [Line Items]
Ownership percentage by parent							100.00%
SOUTHERN POWER CO | Scenario, Forecast | Skookumchuck
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW) | MW	136
PPA Contract Period	20 years
SOUTHERN POWER CO | Scenario, Forecast | Garland Solar Storage
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW) | MW				88
PPA Contract Period				20 years
SOUTHERN POWER CO | Scenario, Forecast | Tranquillity Solar Storage
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW) | MW				72
PPA Contract Period				20 years