DERIVATIVES - Narrative (Details) MWh in Millions, MMBTU in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) MMBTU MWh	Dec. 31, 2019 USD ($)
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	12
Cash flow hedge gain (loss) to be reclassified within twelve months | $	$ (25,000,000)
Collateral already posted, assets, aggregate fair value | $	$ 70,000,000
SOUTHERN POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	4
ALABAMA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	3
GEORGIA POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	4
MISSISSIPPI POWER CO
Derivative [Line Items]
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU	1
Parent Company and Southern Power
Derivative [Line Items]
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months | $	$ (14,000,000)
Registrants | Derivative Counterparties
Derivative [Line Items]
Collateral already posted, assets, aggregate fair value | $	0
SOUTHERN Co GAS
Derivative [Line Items]
Collateral already posted, assets, aggregate fair value | $	$ 70,000,000	$ 99,000,000
Public Utilities, Inventory, Power Position | SOUTHERN POWER CO
Derivative [Line Items]
Net Purchased mmBtu | MWh	1