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Derivatives - Energy-Related and Interest Rate Derivatives (Details)
MMBTU in Millions
 6 Months Ended
Jun. 30, 2018
USD ($)
MMBTU
Interest rate derivatives  
Interest rate derivatives  
Notional Amount $ 2,500,000,000
Fair Value Gain (Loss) at June 30, 2018 $ (69,000,000)
Southern Company | Energy-related, Natural Gas  
Energy-related derivative contracts  
Net Purchased mmBtu | MMBTU 701
Longest Hedge Date 2022
Longest Non-Hedge Date 2029
Southern Company | Interest rate derivatives | 2020-06-15 | Fair Value Hedges of Existing Debt  
Interest rate derivatives  
Notional Amount $ 300,000,000
Interest Rate Received 2.75%
Fair Value Gain (Loss) at June 30, 2018 $ (6,000,000)
Southern Company | Interest rate derivatives | 2020-06-15 | 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 | 7/1/2021 | 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 June 30, 2018 $ (57,000,000)
Southern Company | Interest rate derivatives | 7/1/2021 | 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 82
Longest Hedge Date 2022
GEORGIA POWER CO | Energy-related, Natural Gas  
Energy-related derivative contracts  
Net Purchased mmBtu | MMBTU 174
Longest Hedge Date 2022
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 June 30, 2018 $ (3,000,000)
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 June 30, 2018 $ (3,000,000)
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 13
Longest Hedge Date 2020
MISSISSIPPI POWER CO | Energy-related, Natural Gas  
Energy-related derivative contracts  
Net Purchased mmBtu | MMBTU 66
Longest Hedge Date 2022
SOUTHERN POWER CO | Energy-related, Natural Gas  
Energy-related derivative contracts  
Net Purchased mmBtu | MMBTU 14
Longest Hedge Date 2020
SOUTHERN Co GAS | Energy-related, Natural Gas  
Energy-related derivative contracts  
Net Purchased mmBtu | MMBTU 352
Longest Hedge Date 2020
Longest Non-Hedge Date 2029
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 3,900
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 3,600