SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Total AFUDC and Interest Capitalized (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 327	$ 282	$ 230
Alabama Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	90	68	61
Georgia Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	213	190	138
Mississippi Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	0	0	1
Southern Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	0	6	11
Southern Company Gas
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 24	$ 18	$ 18