SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Average AFUDC Composite Rates (Details)	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Alabama Power
Capitalized Contract Cost [Line Items]
Composite rate	8.10%	7.90%	7.90%
Georgia Power
Capitalized Contract Cost [Line Items]
Composite rate	7.60%	7.30%	7.20%
Mississippi Power
Capitalized Contract Cost [Line Items]
Composite rate	0.00%	5.30%	2.50%
Atlanta Gas Light
Capitalized Contract Cost [Line Items]
Composite rate	7.40%	7.60%	7.70%
Chattanooga Gas
Capitalized Contract Cost [Line Items]
Composite rate	7.10%	7.10%	7.10%
Nicor Gas
Capitalized Contract Cost [Line Items]
Composite rate	4.60%	2.00%	0.10%