Derivatives - Cumulative Basis Adjustments for Fair Value Hedges (Details) - Fair Value Hedging - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Securities due within one year
Derivative [Line Items]
Carrying Amount of the Hedged Item	$ (499)	$ (746)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	1	3
Securities due within one year | GEORGIA POWER CO
Derivative [Line Items]
Carrying Amount of the Hedged Item	(499)	(746)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	1	3
Long-term debt
Derivative [Line Items]
Carrying Amount of the Hedged Item	(2,526)	(2,553)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	65	35
Long-term debt | GEORGIA POWER CO
Derivative [Line Items]
Carrying Amount of the Hedged Item	(497)	(498)
Cumulative Amount of Fair Value Hedging Adjustment included in Carrying Amount of the Hedged Item	$ 2	$ 1