Derivatives - Energy-Related and Interest Rate Derivatives (Details) MMBTU in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) MMBTU
Energy-related, Natural Gas | ALABAMA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	82
Longest Hedge Date	2022
Energy-related, Natural Gas | GEORGIA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	165
Longest Hedge Date	2022
Energy-related, Natural Gas | GULF POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	9
Longest Hedge Date	2020
Energy-related, Natural Gas | MISSISSIPPI POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	69
Longest Hedge Date	2022
Energy-related, Natural Gas | SOUTHERN POWER CO
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	15
Longest Hedge Date	2020
Energy-related, Natural Gas | SOUTHERN Co GAS
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	255
Longest Hedge Date	2021
Longest Non-Hedge Date	2029
Energy-related, Natural Gas | SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Long
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	4,300
Energy-related, Natural Gas | SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Short
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	4,000
Energy-related, Natural Gas | Southern Company
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	595
Longest Hedge Date	2022
Longest Non-Hedge Date	2029
Interest rate derivatives
Interest rate derivatives
Notional Amount	$ 2,500,000,000
Fair Value Gain (Loss) at September 30, 2018	(72,000,000)
Interest rate derivatives | GEORGIA POWER CO | December 2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 500,000,000
Interest Rate Received	1.95%
Fair Value Gain (Loss) at September 30, 2018	$ (3,000,000)
Interest rate derivatives | GEORGIA POWER CO | 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 September 30, 2018	$ (3,000,000)
Interest rate derivatives | Southern Company | 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 September 30, 2018	$ (6,000,000)
Interest rate derivatives | Southern Company | 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 September 30, 2018	$ (60,000,000)
LIBOR | Interest rate derivatives | GEORGIA POWER CO | December 2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.76%
LIBOR | Interest rate derivatives | GEORGIA POWER CO | December 2019 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	2.46%
LIBOR | Interest rate derivatives | Southern Company | June 2020 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.92%
LIBOR | Interest rate derivatives | Southern Company | July 2021 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.87%