DERIVATIVES - Interest Rate Derivatives (Details) - Interest rate derivatives $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Derivative [Line Items]
Notional Amount	$ 1,560
Fair Value Gain (Loss) at September 30, 2020	20
MISSISSIPPI POWER CO | Cash Flow Hedges of Existing Debt
Derivative [Line Items]
Notional Amount	$ 60
Weighted Average Interest Rate Paid	0.58%
Fair Value Gain (Loss) at September 30, 2020	$ 0
Southern Company | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional Amount	$ 1,500
Interest Rate Received	2.35%
Fair Value Gain (Loss) at September 30, 2020	$ 20
London Interbank Offered Rate (LIBOR) | Southern Company | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Weighted Average Interest Rate Paid	0.87%