Derivatives - Schedule of Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - Long-term debt - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
Carrying Amount of the Hedged Item	$ (3,085)	$ (3,024)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	193	235
SOUTHERN Co GAS
Derivative [Line Items]
Carrying Amount of the Hedged Item	(440)	(427)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	$ 57	$ 70