Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Jun. 30, 2018 USD ($)
Southern Company
Long-term debt, including securities due within one year:
Carrying Amount	$ 45,806
Fair Value	46,481
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	8,119
Fair Value	8,435
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	10,294
Fair Value	10,499
GULF POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	1,285
Fair Value	1,311
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	1,779
Fair Value	1,760
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying Amount	5,037
Fair Value	5,094
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying Amount	5,823
Fair Value	$ 5,957