Summary of Significant Accounting Policies - Schedule of AOCI (Loss) Balances, Net of Tax Effects (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ 21,982	$ 25,528	$ 26,612	$ 21,982
Adjustment to beginning balance						$ (40)
Current period change			26	(9)	(50)
Ending balance	$ 26,612		29,039	25,528	26,612
Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(119)
Ending balance				(119)
Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Adjustment to beginning balance						(26)
Current period change			24
Ending balance			(121)
Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(70)
Adjustment to beginning balance						(14)
Current period change			2
Ending balance			(82)	(70)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(130)	(189)	(180)	(130)
Ending balance	(180)		(203)	(189)	(180)
Southern Power
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		3,264	6,498	5,675	3,264
Adjustment to beginning balance						4
Current period change			14	(10)	31
Ending balance	5,675		7,284	6,498	5,675
Southern Power \| Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			25
Ending balance				25
Southern Power \| Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Adjustment to beginning balance						4
Current period change			7
Ending balance			36
Southern Power \| Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(27)
Adjustment to beginning balance						0
Current period change			7
Ending balance			(20)	(27)
Southern Power \| Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		4	(2)	35	4
Ending balance	35		16	(2)	35
Southern Company Gas
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	8,001		9,022	9,109
Adjustment to beginning balance						4
Current period change	26		2	(5)
Ending balance	9,109	8,001	8,570	9,022	9,109
Southern Company Gas \| Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(6)
Ending balance				(6)
Southern Company Gas \| Qualifying Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Adjustment to beginning balance						(1)
Current period change			4
Ending balance			(3)
Southern Company Gas \| Pension and Other Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			26
Adjustment to beginning balance						$ 5
Current period change			(2)
Ending balance			29	26
Southern Company Gas \| Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0		20	26
Ending balance	$ 26	$ 0	$ 26	$ 20	$ 26