Derivatives - Schedule of Interest Rate Derivatives (Details) - Interest rate derivatives $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Derivative [Line Items]
Notional Amount	$ 2,050
Fair Value Gain (Loss) at September 30, 2021	$ 8
ALABAMA POWER CO | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Weighted Average Interest Rate Paid	1.91%
ALABAMA POWER CO | Fair Value Hedges of Existing Debt | March 2028
Derivative [Line Items]
Notional Amount	$ 150
Interest Rate Received	0.00%
Fair Value Gain (Loss) at September 30, 2021	$ 5
Southern Company parent | Fair Value Hedges of Existing Debt | March 2028
Derivative [Line Items]
Notional Amount	$ 400
Interest Rate Received	1.75%
Fair Value Gain (Loss) at September 30, 2021	$ (2)
Southern Company parent | Fair Value Hedges of Existing Debt | April 2030
Derivative [Line Items]
Notional Amount	$ 1,000
Interest Rate Received	3.70%
Fair Value Gain (Loss) at September 30, 2021	$ 3
SOUTHERN Co GAS | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Notional Amount	$ 500
Interest Rate Received	1.75%
Fair Value Gain (Loss) at September 30, 2021	$ 2
London Interbank Offered Rate (LIBOR) | Southern Company parent | Fair Value Hedges of Existing Debt | March 2028
Derivative [Line Items]
Weighted Average Interest Rate Paid	0.68%
London Interbank Offered Rate (LIBOR) | Southern Company parent | Fair Value Hedges of Existing Debt | April 2030
Derivative [Line Items]
Weighted Average Interest Rate Paid	2.36%
London Interbank Offered Rate (LIBOR) | SOUTHERN Co GAS | Fair Value Hedges of Existing Debt
Derivative [Line Items]
Weighted Average Interest Rate Paid	0.38%