SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Total AFUDC and Interest Capitalized (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 282	$ 230	$ 202
Alabama Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	68	61	71
Georgia Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	190	138	103
Mississippi Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	0	1	0
Southern Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	6	11	15
Southern Company Gas
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 18	$ 18	$ 13