Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Billions	Sep. 30, 2024 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 62.6
Fair value	59.8
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	11.2
Fair value	10.3
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	17.4
Fair value	16.3
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1.7
Fair value	1.5
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	2.7
Fair value	2.7
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	8.4
Fair value	$ 7.7