Derivatives - Schedule of Interest Rate Derivatives (Details) - Interest rate derivatives $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Derivative [Line Items]
Notional Amount	$ 1,560
Fair Value Gain (Loss) at March 31, 2021	10
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 March 31, 2021	$ 0
Southern Company parent | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional Amount	$ 1,500
Interest Rate Received	2.35%
Fair Value Gain (Loss) at March 31, 2021	$ 10
London Interbank Offered Rate (LIBOR) | Southern Company parent | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Weighted Average Interest Rate Paid	0.87%