Derivatives - Pre-tax Effects of Derivatives Not Designated as Hedging (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	$ 81	$ 2
Energy Related Derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	47	0
Energy Related Derivative [Member] | Natural Gas Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	50	0
Energy Related Derivative [Member] | Cost of Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(3)	0
Southern Company Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	82
Southern Company Gas [Member] | Energy Related Derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	47
Southern Company Gas [Member] | Energy Related Derivative [Member] | Natural Gas Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	50
Southern Company Gas [Member] | Energy Related Derivative [Member] | Cost of Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(3)
Southern Company Gas [Member] | Weather Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	14
Southern Company Gas [Member] | Predecessor [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net		(5)
Southern Company Gas [Member] | Predecessor [Member] | Energy Related Derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net		19
Southern Company Gas [Member] | Predecessor [Member] | Energy Related Derivative [Member] | Natural Gas Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net		20
Southern Company Gas [Member] | Predecessor [Member] | Energy Related Derivative [Member] | Cost of Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net		(1)
Southern Company Gas [Member] | Predecessor [Member] | Weather Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net		$ 3
Southern Company [Member] | Weather Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	$ 14