FAIR VALUE MEASUREMENTS - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Jun. 30, 2020 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 46,513
Fair value	53,079
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	8,519
Fair value	10,008
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	12,720
Fair value	15,092
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,404
Fair value	1,571
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,097
Fair value	4,423
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	5,827
Fair value	$ 6,817