DERIVATIVES - Interest Rate Derivatives (Details) - Interest rate derivatives $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Derivative [Line Items]
Notional Amount	$ 1,760
Fair Value Gain (Loss) at June 30, 2020	0
SOUTHERN Co GAS | Cash Flow Hedges of Forecasted Debt
Derivative [Line Items]
Notional Amount	$ 200
Weighted Average Interest Rate Paid	1.81%
Fair Value Gain (Loss) at June 30, 2020	$ (23)
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 June 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 June 30, 2020	$ 23
London Interbank Offered Rate (LIBOR) | Southern Company | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Weighted Average Interest Rate Paid	0.87%