Derivatives - Schedule of Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Securities due within one year
Derivative [Line Items]
Carrying Amount of the Hedged Item	$ 0	$ (1,509)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	0	(10)
Long-term debt
Derivative [Line Items]
Carrying Amount of the Hedged Item	(3,320)	0
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	0	0
Long-term debt | SOUTHERN Co GAS
Derivative [Line Items]
Carrying Amount of the Hedged Item	(497)	0
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	$ (1)	$ 0