CONTINGENCIES, COMMITMENTS, AND GUARANTEES - Environmental Remediation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) site state	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Accrued environmental remediation	$ 216	$ 234
Georgia Power
Environmental Exit Cost [Line Items]
Costs recovered annually	12	2	$ 2
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current	15	15
Accrued environmental remediation	0	0
Under recovered environmental remediation costs, other regulatory assets, current	12	12
Under recovered environmental remediation costs, other regulatory assets, deferred	29	40
Southern Company
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current	44	51
Accrued environmental remediation	216	234
Under recovered environmental remediation costs, other regulatory assets, current	46	49
Under recovered environmental remediation costs, other regulatory assets, deferred	265	300
Southern Company Gas
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current	29	36
Accrued environmental remediation	216	233
Under recovered environmental remediation costs, other regulatory assets, current	34	37
Under recovered environmental remediation costs, other regulatory assets, deferred	$ 236	$ 260
Southern Company Gas | Manufactured Gas Plants
Environmental Exit Cost [Line Items]
Number of sites | site	40
Number of states in which subject to environmental remediation liabilities | state	4