Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2018 USD ($)
Southern Company
Long-term debt, including securities due within one year:
Carrying Amount	$ 45,524
Fair Value	45,500
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	8,120
Fair Value	8,321
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	10,227
Fair Value	10,159
GULF POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	1,285
Fair Value	1,290
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	1,736
Fair Value	1,702
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	5,029
Fair Value	5,058
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying Amount	5,908
Fair Value	$ 5,935