DERIVATIVES - Pre-Tax Effects of Derivatives Designated as Cash Flow Hedging (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	$ (154)
Gain (Loss) Recognized in OCI on Derivative		$ (62)	$ 91
Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(13)
Gain (Loss) Recognized in OCI on Derivative		17	(47)
Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(57)
Gain (Loss) Recognized in OCI on Derivative		(1)	(2)
Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(84)
Gain (Loss) Recognized in OCI on Derivative		(78)	140
Georgia Power
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(59)
Gain (Loss) Recognized in OCI on Derivative		0	1
Southern Power
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(88)
Gain (Loss) Recognized in OCI on Derivative		(68)	102
Southern Power | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(4)
Gain (Loss) Recognized in OCI on Derivative		10	(38)
Southern Power | Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(84)
Gain (Loss) Recognized in OCI on Derivative		(78)	140
Southern Company Gas
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(7)
Gain (Loss) Recognized in OCI on Derivative		7	(9)
Southern Company Gas | Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(9)
Gain (Loss) Recognized in OCI on Derivative		7	(9)
Southern Company Gas | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	$ 2
Gain (Loss) Recognized in OCI on Derivative		$ 0	$ 0