Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Mar. 31, 2023 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 56,100.0
Fair value	51,600.0
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	10,600.0
Fair value	9,500.0
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	14,900.0
Fair value	13,700.0
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,500.0
Fair value	1,300.0
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	3,000.0
Fair value	2,900.0
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	7,500.0
Fair value	$ 6,700.0