SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Total AFUDC and Interest Capitalized (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 400	$ 327	$ 282
Alabama Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	109	90	68
Georgia Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	251	213	190
Southern Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	3	0	6
Southern Company Gas
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 37	$ 24	$ 18