Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2019 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 45,182
Fair value	49,424
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	8,516
Fair value	9,646
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	11,787
Fair value	13,001
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,616
Fair value	1,717
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,960
Fair value	5,290
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	5,755
Fair value	$ 6,489