Income Taxes - Narrative (Details)		1 Months Ended	3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Apr. 03, 2018	Sep. 30, 2018 USD ($)	Dec. 31, 2018 operating_facility	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2019	Dec. 31, 2017 USD ($)
Income Tax Contingency [Line Items]
Valuation allowances		$ 190,000,000		$ 190,000,000				$ 190,000,000			$ 148,000,000
Effective tax rate decrease								14.00%
Effective tax (benefit) rate								22.70%	42.60%
Increase in effective tax rate due to loss on Kemper IGCC									$ 3,100,000,000
Impairment charges				36,000,000		$ 0		$ 197,000,000	0
Income taxes benefit				(623,000,000)		(590,000,000)		(598,000,000)	$ (317,000,000)
Unrecognized tax benefits		0		0				$ 0
Significantly increase (decrease) in amount of unrecognized tax benefits								12 months
State
Income Tax Contingency [Line Items]
Valuation allowances		171,000,000		171,000,000				$ 171,000,000
ALABAMA POWER CO
Income Tax Contingency [Line Items]
Effective tax (benefit) rate								23.90%	39.90%
Income taxes benefit				(127,000,000)		(216,000,000)		$ (272,000,000)	$ (493,000,000)
GEORGIA POWER CO
Income Tax Contingency [Line Items]
Valuation allowances		39,000,000		39,000,000				$ 39,000,000
Effective tax (benefit) rate								25.50%	37.00%
Income taxes benefit				(262,000,000)		(350,000,000)		$ (212,000,000)	$ (705,000,000)
GEORGIA POWER CO | Plant Vogtle Units 3 And 4
Income Tax Contingency [Line Items]
Estimated project costs					$ 1,100,000,000
GEORGIA POWER CO | State
Income Tax Contingency [Line Items]
Valuation allowances		33,000,000		33,000,000				$ 33,000,000
Effective tax (benefit) rate	6.00%
GEORGIA POWER CO | Scenario, Forecast | State
Income Tax Contingency [Line Items]
Effective tax (benefit) rate										5.75%
GULF POWER CO
Income Tax Contingency [Line Items]
Effective tax (benefit) rate								(0.50%)	39.40%
Income taxes benefit				4,000,000		(40,000,000)		$ 1,000,000	$ (78,000,000)
MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Valuation allowances		124,000,000		124,000,000				$ 124,000,000
Effective tax (benefit) rate								20.80%	(30.30%)
Increase in effective tax rate due to loss on Kemper IGCC								$ 3,100,000,000
Income taxes benefit				(14,000,000)		(24,000,000)		(23,000,000)	$ 885,000,000
MISSISSIPPI POWER CO | State
Income Tax Contingency [Line Items]
Valuation allowances		124,000,000		124,000,000				$ 124,000,000
SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Effective tax (benefit) rate								(220.30%)	(66.50%)
Impairment charges				36,000,000		0		$ 155,000,000	$ 0
Income taxes benefit				38,000,000		39,000,000	$ 54,000,000	210,000,000	$ 129,000,000
SOUTHERN POWER CO | State
Income Tax Contingency [Line Items]
Income taxes benefit		11,000,000
SOUTHERN POWER CO | Scenario, Forecast
Income Tax Contingency [Line Items]
Number of operating wind facilities | operating_facility			8
SOUTHERN POWER CO | Florida Plants | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Tax Contingency [Line Items]
Pre-tax impairment charge								119,000,000
Impairment charges								36,000,000
SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Valuation allowances		12,000,000		12,000,000				$ 12,000,000
Effective tax (benefit) rate								61.80%	43.40%
Income taxes benefit				(316,000,000)		$ (52,000,000)		$ (475,000,000)	$ (233,000,000)
SOUTHERN Co GAS | State
Income Tax Contingency [Line Items]
Valuation allowances		1,000,000		1,000,000				1,000,000
Investment Tax Credit Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward, amount		$ 2,400,000,000		$ 2,400,000,000				$ 2,400,000,000			$ 2,100,000,000