Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2021 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 51,900.0
Fair value	57,600.0
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	9,100.0
Fair value	10,400.0
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	13,600.0
Fair value	15,200.0
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,600.0
Fair value	1,700.0
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,000.0
Fair value	4,400.0
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	6,800.0
Fair value	$ 7,800.0