FAIR VALUE MEASUREMENTS - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2020 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 49,743
Fair value	56,739
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	9,113
Fair value	10,741
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	12,700
Fair value	15,052
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,402
Fair value	1,562
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,155
Fair value	4,535
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	6,461
Fair value	$ 7,640