Introduction - Other Intangible Assets Subject to Amortization (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ (225)	$ (186)
Total other intangible assets - net	674	873
SOUTHERN POWER CO
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	(66)	(47)
SOUTHERN POWER CO | PPA fair value adjustments
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(66)	(47)
Other Intangible Assets, Net	390	409
SOUTHERN Co GAS
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	246	400
Accumulated Amortization	(133)	(120)
Other Intangible Assets, Net	113	280
Total other intangible assets - net	113	280
SOUTHERN Co GAS | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	156	221
Accumulated Amortization	(78)	(77)
Other Intangible Assets, Net	78	144
SOUTHERN Co GAS | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	26	115
Accumulated Amortization	(6)	(9)
Other Intangible Assets, Net	20	106
SOUTHERN Co GAS | Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(49)	(34)
Other Intangible Assets, Net	15	30
Southern Company
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	824	984
Accumulated Amortization	(225)	(186)
Other Intangible Assets, Net	599	798
Total other intangible assets - gross	899	1,059
Total other intangible assets - net	674	873
Southern Company | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	223	288
Accumulated Amortization	(87)	(83)
Other Intangible Assets, Net	136	205
Southern Company | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	70	159
Accumulated Amortization	(18)	(17)
Other Intangible Assets, Net	52	142
Southern Company | Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(49)	(34)
Other Intangible Assets, Net	15	30
Southern Company | PPA fair value adjustments
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(66)	(47)
Other Intangible Assets, Net	390	409
Southern Company | Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	11	17
Accumulated Amortization	(5)	(5)
Other Intangible Assets, Net	$ 6	$ 12