Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Mar. 31, 2021 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 50,035
Fair value	54,113
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	8,866
Fair value	9,821
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	13,220
Fair value	14,291
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,398
Fair value	1,507
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,030
Fair value	4,347
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	6,588
Fair value	$ 7,264