Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details)	Jun. 30, 2021 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 50,400,000,000
Fair value	56,200,000,000
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	9,300,000,000
Fair value	10,600,000,000
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	13,500,000,000
Fair value	15,200,000,000
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1,900,000,000
Fair value	2,100,000,000
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	4,000,000,000.0
Fair value	4,500,000,000
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	6,300,000,000
Fair value	$ 7,300,000,000