Derivatives - Energy-Related and Interest Rate Derivatives (Details) MMBTU in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) MMBTU
Interest rate derivatives
Interest rate derivatives
Notional Amount	$ 2,750,000,000
Fair Value Gain (Loss) at March 31, 2018	$ (62,000,000)
Southern Company | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	670
Longest Hedge Date	2021
Longest Non-Hedge Date	2026
Southern Company | Interest rate derivatives | 2020-06-01 | 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, 2018	$ (5,000,000)
Southern Company | Interest rate derivatives | 2020-06-01 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.92%
Southern Company | Interest rate derivatives | 7/31/2021 | 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, 2018	$ (50,000,000)
Southern Company | Interest rate derivatives | 7/31/2021 | London Interbank Offered Rate (LIBOR) | 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	74
Longest Hedge Date	2021
GEORGIA POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	167
Longest Hedge Date	2021
GEORGIA POWER CO | Interest rate derivatives | 6/1/2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 250,000,000
Interest Rate Received	5.40%
Fair Value Gain (Loss) at March 31, 2018	$ (1,000,000)
GEORGIA POWER CO | Interest rate derivatives | 6/1/2018 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	4.02%
GEORGIA POWER CO | Interest rate derivatives | 12/1/2018 | Fair Value Hedges of Existing Debt
Interest rate derivatives
Notional Amount	$ 500,000,000
Interest Rate Received	1.95%
Fair Value Gain (Loss) at March 31, 2018	$ (4,000,000)
GEORGIA POWER CO | Interest rate derivatives | 12/1/2018 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	0.76%
GEORGIA POWER CO | Interest rate derivatives | 12/1/2019 | 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, 2018	$ (2,000,000)
GEORGIA POWER CO | Interest rate derivatives | 12/1/2019 | London Interbank Offered Rate (LIBOR) | Fair Value Hedges of Existing Debt
Interest rate derivatives
Weighted Average Interest Rate Paid	2.46%
GULF POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	17
Longest Hedge Date	2020
MISSISSIPPI POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	59
Longest Hedge Date	2021
SOUTHERN POWER CO | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	14
Longest Hedge Date	2020
Longest Non-Hedge Date	2018
SOUTHERN Co GAS | Energy-related, Natural Gas
Energy-related derivative contracts
Net Purchased mmBtu | MMBTU	339
Longest Hedge Date	2020
Longest Non-Hedge Date	2026
SOUTHERN Co GAS | Energy-related, Natural Gas | Long | Derivatives not designated as hedging instruments
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,600
SOUTHERN Co GAS | Energy-related, Natural Gas | Short | Derivatives not designated as hedging instruments
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume | MMBTU	3,300