FINANCING - Notes Payable (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Amount Outstanding	$ 609	$ 2,055
Weighted Average Interest Rate	0.30%	2.10%
Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 609	$ 1,705
Weighted Average Interest Rate	0.30%	2.10%
Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 350
Weighted Average Interest Rate	0.00%	2.30%
Georgia Power
Short-term Debt [Line Items]
Amount Outstanding	$ 60	$ 365
Weighted Average Interest Rate	0.30%	2.20%
Georgia Power | Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 60	$ 115
Weighted Average Interest Rate	0.30%	2.10%
Georgia Power | Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 250
Weighted Average Interest Rate	0.00%	2.20%
Mississippi Power
Short-term Debt [Line Items]
Amount Outstanding	$ 25	$ 0
Mississippi Power | Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 25	$ 0
Weighted Average Interest Rate	0.40%	0.00%
Southern Power
Short-term Debt [Line Items]
Amount Outstanding	$ 175	$ 549
Weighted Average Interest Rate	0.30%	2.20%
Southern Power | Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 175	$ 449
Weighted Average Interest Rate	0.30%	2.10%
Southern Power | Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 100
Weighted Average Interest Rate	0.00%	2.60%
Southern Company Gas
Short-term Debt [Line Items]
Amount Outstanding	$ 324	$ 650
Southern Company Gas | Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 324	$ 650
Weighted Average Interest Rate	0.20%	2.00%
Southern Company Gas Capital | Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 220	$ 372
Weighted Average Interest Rate	0.30%	2.10%
Nicor Gas | Commercial paper
Short-term Debt [Line Items]
Amount Outstanding	$ 104	$ 278
Weighted Average Interest Rate	0.20%	1.80%