Fair Value Measurements - Schedule of Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Billions	Mar. 31, 2026 USD ($)
Long-term debt, including securities due within one year:
Carrying amount	$ 72.1
Fair value	66.5
ALABAMA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	12.0
Fair value	10.7
GEORGIA POWER CO
Long-term debt, including securities due within one year:
Carrying amount	21.7
Fair value	20.2
MISSISSIPPI POWER CO
Long-term debt, including securities due within one year:
Carrying amount	1.8
Fair value	1.7
SOUTHERN POWER CO
Long-term debt, including securities due within one year:
Carrying amount	2.9
Fair value	2.8
SOUTHERN Co GAS
Long-term debt, including securities due within one year:
Carrying amount	9.3
Fair value	$ 8.3