DERIVATIVES - Energy-Related Derivatives (Details) - Energy-related, Natural Gas MMBTU in Millions	Sep. 30, 2020 MMBTU
Energy-related derivative contracts
Net Purchased mmBtu	877
ALABAMA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	78
GEORGIA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	131
MISSISSIPPI POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	86
SOUTHERN POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	14
SOUTHERN Co GAS
Energy-related derivative contracts
Net Purchased mmBtu	568
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Long
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume	4,600
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Short
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume	4,100