SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Total AFUDC and Interest Capitalized (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 202	$ 210	$ 249
Alabama Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	71	84	54
Georgia Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	103	94	63
Mississippi Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	0	0	72
Southern Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	15	17	11
Southern Company Gas
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 13	$ 14	$ 19