Derivatives - Derivative Financial Statement Presentation and Amounts With Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivatives, assets	$ 492	$ 837
Derivatives, liabilities	654	1,036
Derivative asset, Gross amounts offset	(333)	(524)
Derivative liability, Gross amounts offset	(523)	(801)
Derivative asset	159	313
Derivative liability	131	235
Collateral already posted, aggregate fair value	190
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Derivatives, assets	430	741
Derivatives, liabilities	560	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	259	561
Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	291	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	171	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	269	325
Derivatives designated as hedging instruments for regulatory purposes | Derivatives designated as hedging instruments for regulatory purposes | Gain (loss) on energy-related cash flow hedges
Derivatives, Fair Value [Line Items]
Derivatives, assets	23	17
Derivatives, liabilities	31	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	12	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	10	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	11	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	21	26
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	39	79
Derivatives, liabilities	63	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	1	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	3	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	20	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	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 deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	15	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 | Under recovered fuel balance
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	38	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	0	0
ALABAMA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	6	6
Derivatives, liabilities	7	10
Derivative asset, Gross amounts offset	(5)	(4)
Derivative liability, Gross amounts offset	(5)	(4)
Derivative asset	1	2
Derivative liability	2	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	6	6
Derivatives, liabilities	7	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	2	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	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 deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	5	6
GEORGIA POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	9	6
Derivatives, liabilities	18	23
Derivative asset, Gross amounts offset	(8)	(6)
Derivative liability, Gross amounts offset	(8)	(6)
Derivative asset	1	0
Derivative liability	10	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	9	6
Derivatives, liabilities	16	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	4	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	5	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	5	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	11	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	2	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	2	2
MISSISSIPPI POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	3	3
Derivatives, liabilities	6	9
Derivative asset, Gross amounts offset	(3)	(2)
Derivative liability, Gross amounts offset	(3)	(2)
Derivative asset	0	1
Derivative liability	3	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	3	3
Derivatives, liabilities	6	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	2	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	2	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	1	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	4	6
SOUTHERN POWER CO
Derivatives, Fair Value [Line Items]
Derivatives, assets	39	79
Derivatives, liabilities	27	31
Derivative asset, Gross amounts offset	(1)	(3)
Derivative liability, Gross amounts offset	(1)	(3)
Derivative asset	38	76
Derivative liability	26	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	39	79
Derivatives, liabilities	27	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	1	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 | Under recovered fuel balance
Derivatives, Fair Value [Line Items]
Derivatives, liabilities	2	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	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 | 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 | Under recovered fuel balance
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	38	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	0	0
SOUTHERN Co GAS
Derivatives, Fair Value [Line Items]
Derivatives, assets	434	741
Derivatives, liabilities	562	909
Derivative asset, Gross amounts offset	(316)	(508)
Derivative liability, Gross amounts offset	(506)	(785)
Derivative asset	118	233
Derivative liability	56	124
Collateral already posted, aggregate fair value	190	277
Weather derivative premium	11	8
SOUTHERN Co GAS | Gain (loss) on energy-related cash flow hedges
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	2
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Gain (loss) on energy-related cash flow hedges
Derivatives, Fair Value [Line Items]
Derivatives, assets	430	739
Derivatives, liabilities	560	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	171	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	269	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	259	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	291	574
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
Derivatives, Fair Value [Line Items]
Derivatives, assets	4	2
Derivatives, liabilities	1	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	1	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	3	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	1	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	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 | 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	1	1
Southern Company | 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	0
Southern Company | 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	$ 0	$ 6