INCOME TAXES - Narrative (Details)	1 Months Ended		3 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) operating_facility	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Tax Disclosure [Line Items]
Deferred income tax assets			$ 9,048,000,000	$ 8,680,000,000	$ 9,048,000,000
Income tax benefit				$ (1,798,000,000)	(449,000,000)	$ (142,000,000)
Significantly increase or decrease in the amount of the unrecognized tax benefits associated with a majority of Southern				12 months
Unrecognized tax benefits			0		0	18,000,000	$ 484,000,000
Decrease in unrecognized tax benefits from prior periods					18,000,000	196,000,000
ITC and PTC carryforward
Income Tax Disclosure [Line Items]
Deferred income tax assets			2,730,000,000	$ 2,098,000,000	2,730,000,000
Southern Power
Income Tax Disclosure [Line Items]
Deferred income tax assets			2,631,000,000	$ 1,917,000,000	2,631,000,000
Percentage reduction in tax basis of assets				50.00%
Number of wind facilities | operating_facility	8
Income tax benefit			(75,000,000)	$ 56,000,000	164,000,000	939,000,000
Unrecognized tax benefits			0		0	0	17,000,000
Decrease in unrecognized tax benefits from prior periods					0	17,000,000
Southern Power | ITCs and other credits carryforward
Income Tax Disclosure [Line Items]
Reduction in income tax expense, investment tax credits				734,000,000	5,000,000	0
Southern Power | ITC and PTC carryforward
Income Tax Disclosure [Line Items]
Deferred income tax assets			2,128,000,000	1,445,000,000	2,128,000,000
Southern Power | ITC and PTC carryforward | Nacogdoches Biomass Generating Plant
Income Tax Disclosure [Line Items]
Reduction in income tax expense, investment tax credits				5,000,000	1,000,000	18,000,000
Southern Power | Production Tax Credit Carryforward
Income Tax Disclosure [Line Items]
Effective income tax rate reconciliation, tax credit, production, amount				12,000,000	141,000,000	139,000,000
Georgia Power
Income Tax Disclosure [Line Items]
Deferred income tax assets			3,078,000,000	3,123,000,000	3,078,000,000
Income tax benefit				(472,000,000)	(214,000,000)	(830,000,000)
State investment tax credit carryforward				360,000,000
Georgia Power | ITC and PTC carryforward
Income Tax Disclosure [Line Items]
Deferred income tax assets			430,000,000	435,000,000	430,000,000
Mississippi Power
Income Tax Disclosure [Line Items]
Deferred income tax assets			515,000,000	451,000,000	515,000,000
Income tax benefit				(30,000,000)	102,000,000	532,000,000
Unrecognized tax benefits			0		0	0	$ 465,000,000
Decrease in unrecognized tax benefits from prior periods					0	177,000,000
Mississippi Power | Kemper IGCC
Income Tax Disclosure [Line Items]
Unrecognized tax benefits						176,000,000
Decrease in unrecognized tax benefits from prior periods		$ 36,000,000
Mississippi Power | ITC and PTC carryforward
Income Tax Disclosure [Line Items]
Deferred income tax assets			0	0	0
State and Local Jurisdiction | Southern Power
Income Tax Disclosure [Line Items]
Income tax benefit	$ 65,000,000
State and Local Jurisdiction | Southern Power | ITCs and other credits carryforward
Income Tax Disclosure [Line Items]
Reduction in income tax expense, investment tax credits						32,000,000
State and Local Jurisdiction | Georgia Power
Income Tax Disclosure [Line Items]
Valuation allowance				28,000,000
State and Local Jurisdiction | Georgia Power | ITCs and other credits carryforward
Income Tax Disclosure [Line Items]
Reduction in income tax expense, investment tax credits				51,000,000	21,000,000	37,000,000
Deferred Charges Related To Income Taxes, Current | Other Noncurrent Assets | Southern Power | Unrealized Tax Credits
Income Tax Disclosure [Line Items]
Deferred income tax assets			$ 128,000,000	51,000,000	$ 128,000,000	$ 316,000,000
Mississippi | State and Local Jurisdiction | Mississippi Power
Income Tax Disclosure [Line Items]
Operating loss carryforwards, valuation allowance				32,000,000
Oklahoma | State and Local Jurisdiction | Southern Power
Income Tax Disclosure [Line Items]
Operating loss carryforwards, valuation allowance				16,000,000
Florida | State and Local Jurisdiction | Southern Power
Income Tax Disclosure [Line Items]
Operating loss carryforwards, valuation allowance				$ 11,000,000