SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Average AFUDC Composite Rates (Details)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Alabama Power
Capitalized Contract Cost [Line Items]
Composite rate	7.90%	8.10%	8.40%
Georgia Power
Capitalized Contract Cost [Line Items]
Composite rate	7.20%	6.90%	6.90%
Mississippi Power
Capitalized Contract Cost [Line Items]
Composite rate	2.50%	5.40%	7.30%
Atlanta Gas Light
Capitalized Contract Cost [Line Items]
Composite rate	7.70%	7.70%	7.80%
Chattanooga Gas
Capitalized Contract Cost [Line Items]
Composite rate	7.10%	7.10%	7.10%
Nicor Gas
Capitalized Contract Cost [Line Items]
Composite rate	0.10%	0.70%	2.30%