Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Billions	Sep. 30, 2023 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 58.8
Fair value	50.8
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	11.2
Fair value	9.4
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	15.8
Fair value	13.4
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1.6
Fair value	1.3
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	2.7
Fair value	2.4
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	8.1
Fair value	$ 6.5