Derivatives - Schedule of Energy-Related Derivatives (Details) - Energy-related, Natural Gas MMBTU in Millions	Sep. 30, 2021 MMBTU
Energy-related derivative contracts
Net Purchased mmBtu	336.0
ALABAMA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	75.0
GEORGIA POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	99.0
MISSISSIPPI POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	79.0
SOUTHERN POWER CO
Energy-related derivative contracts
Net Purchased mmBtu	6.0
SOUTHERN Co GAS
Energy-related derivative contracts
Net Purchased mmBtu	77.0
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Long
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume	91.4
SOUTHERN Co GAS | Derivatives not designated as hedging instruments | Short
Energy-related derivative contracts
Derivative nonmonetary notional amount net long short position volume	14.3