Financing - Notes Payable (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount Outstanding	$ 2,915	$ 2,439
Weighted Average Interest Rate	3.10%	1.90%
Commercial Paper
Short-term Debt [Line Items]
Amount Outstanding	$ 1,064	$ 1,832
Weighted Average Interest Rate	3.00%	1.80%
Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 1,851	$ 607
Weighted Average Interest Rate	3.10%	2.30%
Alabama Power | Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 3
Weighted Average Interest Rate	0.00%	3.70%
Georgia Power
Short-term Debt [Line Items]
Amount Outstanding	$ 294	$ 150
Weighted Average Interest Rate	3.10%	2.20%
Georgia Power | Commercial Paper
Short-term Debt [Line Items]
Amount Outstanding	$ 294	$ 0
Weighted Average Interest Rate	3.10%	0.00%
Georgia Power | Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 150
Weighted Average Interest Rate	0.00%	2.20%
Mississippi Power
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 4
Mississippi Power | Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 4
Weighted Average Interest Rate	0.00%	3.80%
Southern Power
Short-term Debt [Line Items]
Amount Outstanding	$ 100	$ 105
Weighted Average Interest Rate	3.10%	2.00%
Southern Power | Commercial Paper
Short-term Debt [Line Items]
Amount Outstanding	$ 0	$ 105
Weighted Average Interest Rate	0.00%	2.00%
Southern Power | Short-term bank debt
Short-term Debt [Line Items]
Amount Outstanding	$ 100	$ 0
Weighted Average Interest Rate	3.10%	0.00%
Southern Company Gas
Short-term Debt [Line Items]
Amount Outstanding	$ 650	$ 1,518
Southern Company Gas | Commercial Paper
Short-term Debt [Line Items]
Amount Outstanding	$ 650	$ 1,518
Weighted Average Interest Rate	3.00%	1.80%
Southern Company Gas Capital | Commercial Paper
Short-term Debt [Line Items]
Amount Outstanding	$ 403	$ 1,243
Weighted Average Interest Rate	3.10%	1.70%
Nicor Gas | Commercial Paper
Short-term Debt [Line Items]
Amount Outstanding	$ 247	$ 275
Weighted Average Interest Rate	3.00%	1.80%