FAIR VALUE MEASUREMENTS - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Mar. 31, 2020 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 45,820
Fair value	49,126
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	8,432
Fair value	9,239
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	12,217
Fair value	14,020
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,413
Fair value	1,433
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,369
Fair value	4,376
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	5,836
Fair value	$ 6,416