SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Average AFUDC Composite Rates (Details)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Alabama Power
Capitalized Contract Cost [Line Items]
Composite rate	8.40%	8.30%	8.30%
Georgia Power
Capitalized Contract Cost [Line Items]
Composite rate	6.90%	7.30%	5.60%
Mississippi Power
Capitalized Contract Cost [Line Items]
Composite rate	7.30%	3.30%	6.70%
Atlanta Gas Light
Capitalized Contract Cost [Line Items]
Composite rate	7.80%	7.90%	8.10%
Chattanooga Gas
Capitalized Contract Cost [Line Items]
Composite rate	7.10%	7.40%	7.40%
Nicor Gas
Capitalized Contract Cost [Line Items]
Composite rate	2.30%	2.10%	1.20%