Derivatives - Energy-Related and Interest Rate Derivatives (Details) MMBTU in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) MMBTU
Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	538
Interest rate derivatives
Interest rate derivatives
Notional Amount | $	$ 2,000,000,000
Fair Value Gain (Loss) at March 31, 2019 | $	(35,000,000)
Southern Company | Interest rate derivatives | June 2020 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount | $	$ 300,000,000
Interest Rate Received	2.75%
Fair Value Gain (Loss) at March 31, 2019 | $	$ (3,000,000)
Southern Company | Interest rate derivatives | July 2021 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount | $	$ 1,500,000,000
Interest Rate Received	2.35%
Fair Value Gain (Loss) at March 31, 2019 | $	$ (30,000,000)
Southern Company | LIBOR | Interest rate derivatives | June 2020 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.92%
Southern Company | LIBOR | Interest rate derivatives | July 2021 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.87%
ALABAMA POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	72
GEORGIA POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	153
GEORGIA POWER CO | Interest rate derivatives | December 2019 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount | $	$ 200,000,000
Interest Rate Received	4.25%
Fair Value Gain (Loss) at March 31, 2019 | $	$ (2,000,000)
GEORGIA POWER CO | LIBOR | Interest rate derivatives | December 2019 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	2.46%
MISSISSIPPI POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	61
SOUTHERN POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	9
SOUTHERN Co GAS | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	243
SOUTHERN Co GAS | Energy-related, Natural Gas | Derivatives not designated as hedging instruments | Long
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,800
SOUTHERN Co GAS | Energy-related, Natural Gas | Derivatives not designated as hedging instruments | Short
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,600