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, 2016	Dec. 31, 2015	Dec. 31, 2014
Energy-related derivatives | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net			$ 38	$ (2)	$ 2
Energy-related derivatives | Wholesale Electric Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net			2	(5)	6
Energy-related derivatives | Fuel [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net			0	3	(4)
Energy-related derivatives | 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			33	0	0
Energy-related derivatives | 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	0
Successor [Member] | Southern Company Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 3
Successor [Member] | Southern Company Gas [Member] | Energy-related derivatives | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	36
Successor [Member] | Southern Company Gas [Member] | Energy-related derivatives | 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	33
Successor [Member] | Southern Company Gas [Member] | Energy-related derivatives | 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
Successor [Member] | Southern Company Gas [Member] | Weather Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 6
Predecessor [Member] | Southern Company Gas [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net		$ (162)		(22)	155
Predecessor [Member] | Southern Company Gas [Member] | Energy-related derivatives | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		(63)		50	142
Predecessor [Member] | Southern Company Gas [Member] | Energy-related derivatives | 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		(1)		56	149
Predecessor [Member] | Southern Company Gas [Member] | Energy-related derivatives | 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		(62)		(6)	(7)
Predecessor [Member] | Southern Company Gas [Member] | Weather Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net		$ 3		$ 12	$ (7)