Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Mar. 31, 2019 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 42,535
Fair value	43,910
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	7,921
Fair value	8,424
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	10,910
Fair value	11,249
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,619
Fair value	1,619
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,995
Fair value	5,131
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	5,928
Fair value	$ 6,176