Introduction - Other Intangibles (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 985	$ 984
Accumulated Amortization	(212)	(186)
Other Intangible Assets, Net	773	798
Total other intangible assets - gross	1,060	1,059
Total other intangible assets - net	848	873
Customer relationships | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	288	288
Accumulated Amortization	(93)	(83)
Other Intangible Assets, Net	195	205
Trade names | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	159	159
Accumulated Amortization	(19)	(17)
Other Intangible Assets, Net	140	142
Storage and transportation contracts | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(40)	(34)
Other Intangible Assets, Net	24	30
PPA fair value adjustments | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(54)	(47)
Other Intangible Assets, Net	402	409
Other | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	18	17
Accumulated Amortization	(6)	(5)
Other Intangible Assets, Net	12	12
Federal Communications Commission licenses | Southern Company
Indefinite-lived Intangible Assets [Line Items]
Other intangible assets not subject to amortization	75	75
SOUTHERN POWER CO
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	(54)	(47)
SOUTHERN POWER CO | PPA fair value adjustments
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	456	456
Accumulated Amortization	(54)	(47)
Other Intangible Assets, Net	402	409
SOUTHERN Co GAS
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	400	400
Accumulated Amortization	(136)	(120)
Other Intangible Assets, Net	264	280
Total other intangible assets - net	264	280
SOUTHERN Co GAS | Customer relationships
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	221	221
Accumulated Amortization	(86)	(77)
Other Intangible Assets, Net	135	144
SOUTHERN Co GAS | Trade names
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	115	115
Accumulated Amortization	(10)	(9)
Other Intangible Assets, Net	105	106
SOUTHERN Co GAS | Storage and transportation contracts
Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(40)	(34)
Other Intangible Assets, Net	$ 24	$ 30