Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2017 USD ($)
Southern Company
Long-term debt:
Carrying Amount	$ 47,269
Fair Value	49,348
ALABAMA POWER CO
Long-term debt:
Carrying Amount	7,404
Fair Value	8,031
GEORGIA POWER CO
Long-term debt:
Carrying Amount	11,713
Fair Value	12,237
GULF POWER CO
Long-term debt:
Carrying Amount	1,292
Fair Value	1,352
MISSISSIPPI POWER CO
Long-term debt:
Carrying Amount	2,123
Fair Value	2,117
SOUTHERN POWER CO
Long-term debt:
Carrying Amount	5,810
Fair Value	5,916
SOUTHERN Co GAS
Long-term debt:
Carrying Amount	5,862
Fair Value	$ 6,230