DERIVATIVES - Derivative Financial Statement Presentation and Amounts With Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	$ 810	$ 696
Derivative asset, gross amount offset	(529)	(463)
Derivative asset, net amounts recognized in the balance sheets	281	233
Derivative liability, gross amounts recognized	621	751
Derivative liability, gross amounts offset	(557)	(562)
Derivative liability, gross amounts recognized in balance sheets	64	189
Collateral already posted, aggregate fair value	28	99
Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	658	668
Derivative liability, gross amounts recognized	563	583
Energy-related derivatives | Other deferred charges and assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	270	207
Energy-related derivatives | Other deferred credits and liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	232	225
Energy-related derivatives | Assets from risk management activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	388	461
Energy-related derivatives | Liabilities from risk management activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	331	358
Alabama Power
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	12	4
Derivative asset, gross amount offset	(7)	(2)
Derivative asset, net amounts recognized in the balance sheets	5	2
Derivative liability, gross amounts recognized	7	24
Derivative liability, gross amounts offset	(7)	(2)
Derivative liability, gross amounts recognized in balance sheets	0	22
Georgia Power
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	15	4
Derivative asset, gross amount offset	(12)	(3)
Derivative asset, net amounts recognized in the balance sheets	3	1
Derivative liability, gross amounts recognized	13	70
Derivative liability, gross amounts offset	(12)	(3)
Derivative liability, gross amounts recognized in balance sheets	1	67
Mississippi Power
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	9	1
Derivative asset, gross amount offset	(7)	(1)
Derivative asset, net amounts recognized in the balance sheets	2	0
Derivative liability, gross amounts recognized	9	27
Derivative liability, gross amounts offset	(7)	(1)
Derivative liability, gross amounts recognized in balance sheets	2	26
Southern Power
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, net amounts recognized in the balance sheets	89	19
Derivative liability, gross amounts recognized in balance sheets	26	27
Southern Power | Energy-related derivatives | Other current assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	0	2
Southern Power | Energy-related derivatives | Other current liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	1	1
Southern Company Gas
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	665	668
Derivative asset, gross amount offset	(503)	(456)
Derivative asset, net amounts recognized in the balance sheets	162	212
Derivative liability, gross amounts recognized	566	596
Derivative liability, gross amounts offset	(531)	(555)
Derivative liability, gross amounts recognized in balance sheets	35	41
Collateral already posted, aggregate fair value	28	99
Southern Company Gas | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	658	666
Derivative liability, gross amounts recognized	562	582
Southern Company Gas | Energy-related derivatives | Other current liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	330	357
Southern Company Gas | Energy-related derivatives | Other deferred charges and assets | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	270	207
Southern Company Gas | Energy-related derivatives | Other deferred credits and liabilities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	232	225
Southern Company Gas | Energy-related derivatives | Assets from risk management activities | Not Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	388	459
Hedging Instruments for Regulatory Purposes | Energy-related derivatives | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	42	9
Derivative liability, gross amounts recognized	30	114
Hedging Instruments for Regulatory Purposes | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	18	6
Hedging Instruments for Regulatory Purposes | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	19	44
Hedging Instruments for Regulatory Purposes | Energy-related derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	24	3
Hedging Instruments for Regulatory Purposes | Energy-related derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	11	70
Hedging Instruments for Regulatory Purposes | Alabama Power | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	12	4
Derivative liability, gross amounts recognized	7	24
Hedging Instruments for Regulatory Purposes | Alabama Power | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	7	2
Hedging Instruments for Regulatory Purposes | Alabama Power | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	2	14
Hedging Instruments for Regulatory Purposes | Alabama Power | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	5	2
Hedging Instruments for Regulatory Purposes | Alabama Power | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	5	10
Hedging Instruments for Regulatory Purposes | Georgia Power | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	15	4
Derivative liability, gross amounts recognized	13	53
Hedging Instruments for Regulatory Purposes | Georgia Power | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	7	1
Hedging Instruments for Regulatory Purposes | Georgia Power | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	5	32
Hedging Instruments for Regulatory Purposes | Georgia Power | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	8	3
Hedging Instruments for Regulatory Purposes | Georgia Power | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	8	21
Hedging Instruments for Regulatory Purposes | Mississippi Power | Energy-related derivatives | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	9	1
Derivative liability, gross amounts recognized	9	27
Hedging Instruments for Regulatory Purposes | Mississippi Power | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	4	0
Hedging Instruments for Regulatory Purposes | Mississippi Power | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	3	15
Hedging Instruments for Regulatory Purposes | Mississippi Power | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	5	1
Hedging Instruments for Regulatory Purposes | Mississippi Power | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	6	12
Hedging Instruments for Regulatory Purposes | Southern Company Gas | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	6	0
Derivative liability, gross amounts recognized	1	10
Hedging Instruments for Regulatory Purposes | Southern Company Gas | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	1	9
Hedging Instruments for Regulatory Purposes | Southern Company Gas | Energy-related derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	0	0
Hedging Instruments for Regulatory Purposes | Southern Company Gas | Energy-related derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	1
Hedging Instruments for Regulatory Purposes | Southern Company Gas | Energy-related derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	6	0
Cash Flow and Fair Value Hedging | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	110	19
Derivative liability, gross amounts recognized	28	54
Cash Flow and Fair Value Hedging | Energy-related derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	3	1
Cash Flow and Fair Value Hedging | Energy-related derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	5	6
Cash Flow and Fair Value Hedging | Interest rate derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Interest rate derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	1
Cash Flow and Fair Value Hedging | Interest rate derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	20	2
Cash Flow and Fair Value Hedging | Interest rate derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	23
Cash Flow and Fair Value Hedging | Foreign currency derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	87	16
Cash Flow and Fair Value Hedging | Foreign currency derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Foreign currency derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Foreign currency derivatives | Liabilities from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	23	24
Cash Flow and Fair Value Hedging | Georgia Power | Interest rate derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Georgia Power | Interest rate derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	17
Cash Flow and Fair Value Hedging | Southern Power | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	89	17
Derivative liability, gross amounts recognized	25	26
Cash Flow and Fair Value Hedging | Southern Power | Energy-related derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	2	1
Cash Flow and Fair Value Hedging | Southern Power | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	2	2
Cash Flow and Fair Value Hedging | Southern Power | Foreign currency derivatives | Other current assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Southern Power | Foreign currency derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	23	24
Cash Flow and Fair Value Hedging | Southern Power | Foreign currency derivatives | Other deferred charges and assets | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	87	16
Cash Flow and Fair Value Hedging | Southern Power | Foreign currency derivatives | Other deferred credits and liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Southern Company Gas | Energy-related derivatives | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	1	2
Derivative liability, gross amounts recognized	3	4
Cash Flow and Fair Value Hedging | Southern Company Gas | Energy-related derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	3	4
Cash Flow and Fair Value Hedging | Southern Company Gas | Energy-related derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	1	0
Cash Flow and Fair Value Hedging | Southern Company Gas | Interest rate derivatives | Other current liabilities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative liability, gross amounts recognized	0	0
Cash Flow and Fair Value Hedging | Southern Company Gas | Interest rate derivatives | Assets from risk management activities | Designated as Hedging Instrument
Fair value of energy-related derivatives and interest rate derivatives
Derivative asset, gross amounts recognized	$ 0	$ 2