Derivatives - Schedule of Derivative Category and Balance Sheet Location (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	$ 636	$ 810
Derivative asset, Gross amounts offset	(450)	(529)
Derivative asset, net amounts recognized in the balance sheets	186	281
Derivative liability, gross amount recognized	545	621
Derivative liability, Gross amounts offset	(477)	(557)
Derivative liability, net amounts recognized in the balance sheets	68	64
Collateral already posted, assets, aggregate fair value	27
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	537	658
Derivative liability, gross amount recognized	502	563
Derivatives not designated as hedging instruments | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	265	388
Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	265	331
Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	272	270
Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	237	232
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	46	42
Derivative liability, gross amount recognized	19	30
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	30	24
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	4	11
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	16	18
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	15	19
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	53	110
Derivative liability, gross amount recognized	24	28
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	2	3
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	1	5
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	10	20
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	0	0
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	0	0
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	23	23
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	41	87
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	0	0
ALABAMA POWER CO
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	12	12
Derivative asset, Gross amounts offset	(4)	(7)
Derivative asset, net amounts recognized in the balance sheets	8	5
Derivative liability, gross amount recognized	4	7
Derivative liability, Gross amounts offset	(4)	(7)
Derivative liability, net amounts recognized in the balance sheets	0	0
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	12	12
Derivative liability, gross amount recognized	4	7
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	7	7
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	0	2
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	5	5
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	4	5
GEORGIA POWER CO
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	17	15
Derivative asset, Gross amounts offset	(7)	(12)
Derivative asset, net amounts recognized in the balance sheets	10	3
Derivative liability, gross amount recognized	8	13
Derivative liability, Gross amounts offset	(7)	(12)
Derivative liability, net amounts recognized in the balance sheets	1	1
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	17	15
Derivative liability, gross amount recognized	8	13
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	11	7
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	2	5
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	6	8
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	6	8
MISSISSIPPI POWER CO
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	11	9
Derivative asset, Gross amounts offset	(5)	(7)
Derivative asset, net amounts recognized in the balance sheets	6	2
Derivative liability, gross amount recognized	6	9
Derivative liability, Gross amounts offset	(5)	(7)
Derivative liability, net amounts recognized in the balance sheets	1	2
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	11	9
Derivative liability, gross amount recognized	6	9
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	6	4
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	1	3
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	5	5
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	5	6
SOUTHERN POWER CO
Derivatives, Fair Value [Line Items]
Derivative asset, net amounts recognized in the balance sheets	42	89
Derivative liability, net amounts recognized in the balance sheets	23	26
SOUTHERN POWER CO | Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	0	0
Derivative liability, gross amount recognized	0	1
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	0	0
SOUTHERN POWER CO | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	0	1
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	42	89
Derivative liability, gross amount recognized	23	25
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	1	2
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	0	2
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	0	0
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	23	23
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	41	87
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Foreign currency derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	0	0
SOUTHERN Co GAS
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	544	665
Derivative asset, Gross amounts offset	(434)	(503)
Derivative asset, net amounts recognized in the balance sheets	110	162
Derivative liability, gross amount recognized	504	566
Derivative liability, Gross amounts offset	(461)	(531)
Derivative liability, net amounts recognized in the balance sheets	43	35
Collateral already posted, assets, aggregate fair value	27	28
SOUTHERN Co GAS | Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	537	658
Derivative liability, gross amount recognized	502	562
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	265	388
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	265	330
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	272	270
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	237	232
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	6	6
Derivative liability, gross amount recognized	1	1
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	6	6
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	1	1
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	1	1
Derivative liability, gross amount recognized	1	3
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivative asset, gross amount recognized	1	1
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative liability, gross amount recognized	$ 1	$ 3