Derivatives - Pre-Tax Effects of Derivatives Not Designated as Hedging (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Energy-related derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net			$ (86)	$ 38	$ (2)
Energy-related derivatives | Wholesale Electric Revenues | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net			(4)	2	(5)
Energy-related derivatives | Fuel | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net			0	0	3
Energy-related derivatives | Natural Gas Revenues | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net			(80)	33	0
Energy-related derivatives | Cost of Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net			(2)	3	0
Weather Derivative
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net			23	6
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	$ 3		24
SOUTHERN Co GAS | Energy-related derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net	36		(82)
SOUTHERN Co GAS | Energy-related derivatives | Natural Gas Revenues | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net	33		(80)
SOUTHERN Co GAS | Energy-related derivatives | Cost of Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net	$ 3		(2)
SOUTHERN Co GAS | Weather Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net			$ 23
Predecessor | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net		$ (162)			(22)
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net		(63)		50
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Natural Gas Revenues | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net		(1)		56
Predecessor | SOUTHERN Co GAS | Energy-related derivatives | Cost of Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging iinstruments, gain (loss), net		(62)		(6)
Predecessor | SOUTHERN Co GAS | Weather Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net		$ 6		$ 3	$ 12