Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Mar. 31, 2022 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 51,600.0
Fair value	51,900.0
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	9,800.0
Fair value	10,000.0
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	13,200.0
Fair value	13,500.0
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,500.0
Fair value	1,500.0
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	3,700.0
Fair value	3,800.0
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	6,900.0
Fair value	$ 6,900.0