Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Jun. 30, 2019 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 42,596
Fair value	45,394
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	7,922
Fair value	8,717
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	10,969
Fair value	11,749
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,618
Fair value	1,657
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	5,011
Fair value	5,261
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	5,916
Fair value	$ 6,420