Derivatives - Pre-Tax Effects of Unrealized Derivative Gains (Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	$ (35)	$ (217)
Regulatory Hedge Unrealized Gain	68	3
Collateral already posted, aggregate fair value	62
Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	8
Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(16)	(130)
Energy-related derivatives | Other regulatory liabilities current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	56	3
Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(19)	(87)
Energy-related derivatives | Other regulatory liabilities deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	12	0
Alabama Power [Member] | Other Regulatory Assets [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(1)	(55)
Alabama Power [Member] | Other Regulatory Liabilities [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	12	1
Alabama Power [Member] | Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(1)	(40)
Alabama Power [Member] | Energy-related derivatives | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	8	1
Alabama Power [Member] | Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	0	(15)
Alabama Power [Member] | Energy-related derivatives | Other regulatory liabilities deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	4	0
Hedging Instruments for Regulatory Purposes [Member] | Georgia Power [Member] | Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	0	(12)
Hedging Instruments for Regulatory Purposes [Member] | Georgia Power [Member] | Energy-related derivatives | Other regulatory liabilities current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	29	2
Hedging Instruments for Regulatory Purposes [Member] | Georgia Power [Member] | Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	0	(3)
Hedging Instruments for Regulatory Purposes [Member] | Georgia Power [Member] | Energy-related derivatives | Other deferred credits and liabilities [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	7	0
Hedging Instruments for Regulatory Purposes [Member] | Georgia Power [Member] | Energy-related derivatives | Other Regulatory Assets [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	0	(15)
Hedging Instruments for Regulatory Purposes [Member] | Georgia Power [Member] | Energy-related derivatives | Other Regulatory Liabilities [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	36	2
Hedging Instruments for Regulatory Purposes [Member] | Gulf Power [Member] | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(25)	(100)
Regulatory Hedge Unrealized Gain	1	0
Hedging Instruments for Regulatory Purposes [Member] | Gulf Power [Member] | Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(9)	(49)
Hedging Instruments for Regulatory Purposes [Member] | Gulf Power [Member] | Energy-related derivatives | Other regulatory liabilities current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	1	0
Hedging Instruments for Regulatory Purposes [Member] | Gulf Power [Member] | Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(16)	(51)
Hedging Instruments for Regulatory Purposes [Member] | Gulf Power [Member] | Energy-related derivatives | Other regulatory liabilities deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	0	0
Hedging Instruments for Regulatory Purposes [Member] | Mississippi Power [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(8)	(47)
Regulatory Hedge Unrealized Gain	1	0
Hedging Instruments for Regulatory Purposes [Member] | Mississippi Power [Member] | Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(5)	(29)
Hedging Instruments for Regulatory Purposes [Member] | Mississippi Power [Member] | Energy-related derivatives | Other regulatory liabilities current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	1	0
Hedging Instruments for Regulatory Purposes [Member] | Mississippi Power [Member] | Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(3)	(18)
Hedging Instruments for Regulatory Purposes [Member] | Mississippi Power [Member] | Energy-related derivatives | Other regulatory liabilities deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	0	0
Successor [Member] | Southern Company Gas [Member]
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	62	96
Successor [Member] | Southern Company Gas [Member] | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	8
Successor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(1)
Regulatory Hedge Unrealized Gain	18
Successor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(1)
Successor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory liabilities current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	17
Successor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	0
Successor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory liabilities deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	1
Predecessor [Member] | Southern Company Gas [Member] | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value		$ 19
Predecessor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(17)
Regulatory Hedge Unrealized Gain	15
Predecessor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory assets current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(15)
Predecessor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory liabilities current [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	15
Predecessor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory assets deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Loss	(2)
Predecessor [Member] | Hedging Instruments for Regulatory Purposes [Member] | Southern Company Gas [Member] | Energy-related derivatives | Other regulatory liabilities deferred [Member]
Derivatives, Fair Value [Line Items]
Regulatory Hedge Unrealized Gain	$ 0