Derivatives - Derivative Financial Statement Presentation and Amounts With Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivatives, assets	$ 489	$ 837
Derivatives, liabilities	771	1,036
Derivative asset, Gross amounts offset	(348)	(524)
Derivative liability, Gross amounts offset	(514)	(801)
Derivative asset	141	313
Derivative liability	257	235
Collateral already posted, aggregate fair value	166
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivatives, assets	476	741
Derivatives, liabilities	553	900
Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	311	561
Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	342	575
Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	165	180
Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	211	325
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	11	17
Derivatives, liabilities	105	55
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	5	8
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	61	23
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	6	9
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	44	26
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Derivatives, assets		0
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Liabilities held for sale, current
Derivatives, Fair Value [Line Items]
Derivatives, liabilities		6
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]
Derivatives, assets	2	79
Derivatives, liabilities	113	81
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	3
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	8	7
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	2
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 assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
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]
Derivatives, liabilities	72	19
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	0
Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	30
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]
Derivatives, assets	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]
Derivatives, liabilities	24	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]
Derivatives, assets	0	75
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]
Derivatives, liabilities	8	0
ALABAMA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	5	6
Derivatives, liabilities	22	10
Derivative asset, Gross amounts offset	(2)	(4)
Derivative liability, Gross amounts offset	(2)	(4)
Derivative asset	3	2
Derivative liability	20	6
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	5	6
Derivatives, liabilities	22	10
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	3
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	12	4
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	3
ALABAMA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	10	6
GEORGIA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	6
Derivatives, liabilities	109	23
Derivative asset, Gross amounts offset	(3)	(6)
Derivative liability, Gross amounts offset	(3)	(6)
Derivative asset	0	0
Derivative liability	106	17
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	6
Derivatives, liabilities	49	21
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	2
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	27	8
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	4
GEORGIA POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	22	13
GEORGIA 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]
Derivatives, assets	0	0
Derivatives, liabilities	60	2
GEORGIA POWER CO | 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 assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
GEORGIA POWER CO | 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]
Derivatives, liabilities	60	2
MISSISSIPPI POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	3
Derivatives, liabilities	24	9
Derivative asset, Gross amounts offset	(2)	(2)
Derivative liability, Gross amounts offset	(2)	(2)
Derivative asset	0	1
Derivative liability	22	7
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	3
Derivatives, liabilities	24	9
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	1
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	12	3
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	2	2
MISSISSIPPI POWER CO | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	12	6
SOUTHERN POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	79
Derivatives, liabilities	36	31
Derivative asset, Gross amounts offset	0	(3)
Derivative liability, Gross amounts offset	0	(3)
Derivative asset	0	76
Derivative liability	36	28
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]
Derivatives, assets	0	79
Derivatives, liabilities	36	31
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	3
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other current liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	4	6
SOUTHERN POWER CO | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	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 | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	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]
Derivatives, assets	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]
Derivatives, liabilities	24	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]
Derivatives, assets	0	75
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]
Derivatives, liabilities	8	0
SOUTHERN Co GAS
Derivatives, Fair Value [Line Items]
Derivatives, assets	477	741
Derivatives, liabilities	573	909
Derivative asset, Gross amounts offset	(341)	(508)
Derivative liability, Gross amounts offset	(507)	(785)
Derivative asset	136	233
Derivative liability	66	124
Collateral already posted, aggregate fair value	166	277
Weather derivative premium		8
SOUTHERN Co GAS | Gain (loss) on energy-related cash flow hedges
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	8
SOUTHERN Co GAS | Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivatives, assets	476	739
Derivatives, liabilities	553	899
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	165	180
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	211	325
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	311	559
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	342	574
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	2
Derivatives, liabilities	10	9
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	0	1
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	1	2
SOUTHERN Co GAS | Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	10	8
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]
Derivatives, assets	0	0
Derivatives, liabilities	10	1
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred charges and assets
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	1	0
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Assets from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, assets	0	0
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	4	1
SOUTHERN Co GAS | 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]
Derivatives, assets	0	0
SOUTHERN Co GAS | Derivatives designated as hedging instruments in cash flow and fair value hedges | Derivatives designated as hedging instruments for regulatory purposes | Interest rate derivatives | Liabilities from risk management activities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	$ 5	$ 0