SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Total AFUDC and Interest Capitalized (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 480	$ 339	$ 400
Alabama Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	88	76	109
Georgia Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	327	209	251
Mississippi Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	0	0	0
Southern Power
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	25	7	3
Southern Company Gas
Capitalized Contract Cost [Line Items]
Total AFUDC and interest capitalized	$ 38	$ 47	$ 37