Derivatives - Pre-Tax Effects of Unrealized Derivative Gains (Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	$ (52)	$ (35)
Regulatory hedge unrealized gain	8	68
Collateral already posted, aggregate fair value	193	62
Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	6	8
Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(34)	(16)
Energy-related derivatives | Other regulatory liabilities current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	7	56
Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(18)	(19)
Energy-related derivatives | Other regulatory liabilities deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	1	12
ALABAMA POWER CO | Other regulatory assets
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(7)	(1)
ALABAMA POWER CO | Other regulatory liabilities
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	1	12
ALABAMA POWER CO | Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(4)	(1)
ALABAMA POWER CO | Energy-related derivatives | Other current liabilities
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	1	8
ALABAMA POWER CO | Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(3)	0
ALABAMA POWER CO | Energy-related derivatives | Other regulatory liabilities deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	4
SOUTHERN Co GAS
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	193	62
SOUTHERN Co GAS | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	6
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(7)	0
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other regulatory liabilities current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	29
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(6)	0
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other deferred credits and liabilities
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	7
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other regulatory assets
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(13)	0
Hedging Instruments for Regulatory Purposes | GEORGIA POWER CO | Energy-related derivatives | Other regulatory liabilities
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	36
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(21)	(25)
Regulatory hedge unrealized gain	0	1
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(14)	(9)
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other regulatory liabilities current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	1
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(7)	(16)
Hedging Instruments for Regulatory Purposes | GULF POWER CO | Energy-related derivatives | Other regulatory liabilities deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	0
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(7)	(8)
Regulatory hedge unrealized gain	0	1
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(5)	(5)
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory liabilities current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	1
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(2)	(3)
Hedging Instruments for Regulatory Purposes | MISSISSIPPI POWER CO | Energy-related derivatives | Other regulatory liabilities deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0	0
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(4)
Regulatory hedge unrealized gain	7
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(4)
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	7
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	0
Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	0
Predecessor | SOUTHERN Co GAS | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value		$ 8
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(1)
Regulatory hedge unrealized gain	18
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	(1)
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities current
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	17
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory assets deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized loss	0
Predecessor | Hedging Instruments for Regulatory Purposes | SOUTHERN Co GAS | Energy-related derivatives | Other regulatory liabilities deferred
Derivatives, Fair Value [Line Items]
Regulatory hedge unrealized gain	$ 1