Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Asset Retirement Obligation, Cash Paid to Settle			$ 177	$ 171	$ 37
Goodwill	$ 6,251		6,268	6,251
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	957		984	957
Accumulated Amortization	(62)		(186)	(62)
Other Intangible Assets, Net	895		798	895
Total other intangible assets, gross	1,032		1,059	1,032
Total other intangible assets, net	970		873	970
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
2018			95
2019			77
2020			65
2021			56
2022			51
Amortization of intangible assets			124	50	$ 3
Customer relationships
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	268		288	268
Accumulated Amortization	(32)		(83)	(32)
Other Intangible Assets, Net	236		205	236
Trade names
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	158		159	158
Accumulated Amortization	(5)		(17)	(5)
Other Intangible Assets, Net	153		142	153
Storage and transportation contracts
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	64		64	64
Accumulated Amortization	(2)		(34)	(2)
Other Intangible Assets, Net	62		30	62
PPA fair value adjustments
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	456		456	456
Accumulated Amortization	(22)		(47)	(22)
Other Intangible Assets, Net	434		409	434
Other
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	11		17	11
Accumulated Amortization	(1)		(5)	(1)
Other Intangible Assets, Net	10		$ 12	$ 10
Minimum | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			11 years
Minimum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			5 years
Minimum | Patents
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			3 years
Minimum | Storage and transportation contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			1 year
Minimum | Software and other
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			1 year
Minimum | PPA fair value adjustments
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			10 years
Maximum | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			26 years
Maximum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			28 years
Maximum | Patents
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			10 years
Maximum | Storage and transportation contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			5 years
Maximum | Software and other
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			12 years
Maximum | PPA fair value adjustments
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			20 years
Utility Plant in Service
Acquired Finite-Lived Intangible Assets [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate			2.90%	3.00%	3.00%
Parent Company | FCC Licenses
Finite-Lived Intangible Assets, Net [Abstract]
Other intangible assets not subject to amortization:	75		$ 75	$ 75
GEORGIA POWER CO
Acquired Finite-Lived Intangible Assets [Line Items]
Asset Retirement Obligation, Cash Paid to Settle			$ 120	$ 123	$ 29
Depreciation of cost of utility plant in service, composite straight-line rate			2.70%	2.80%	2.70%
SOUTHERN Co GAS
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	5,967		$ 5,967	$ 5,967
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	400		400	400
Accumulated Amortization	(34)		(120)	(34)
Other Intangible Assets, Net	366		280	366
Total other intangible assets, net	366		280	366
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
2018			58
2019			40
2020			28
2021			21
2022			17
Amortization of intangible assets	32		54
SOUTHERN Co GAS | Customer relationships
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	221		221	221
Accumulated Amortization	(30)		(77)	(30)
Other Intangible Assets, Net	191		144	191
SOUTHERN Co GAS | Trade names
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	115		115	115
Accumulated Amortization	(2)		(9)	(2)
Other Intangible Assets, Net	113		106	113
SOUTHERN Co GAS | Storage and transportation contracts
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	64		64	64
Accumulated Amortization	(2)		(34)	(2)
Other Intangible Assets, Net	62		$ 30	$ 62
SOUTHERN Co GAS | Minimum | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			11 years
SOUTHERN Co GAS | Minimum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			10 years
SOUTHERN Co GAS | Minimum | Storage and transportation contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			1 year
SOUTHERN Co GAS | Maximum | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			16 years
SOUTHERN Co GAS | Maximum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			28 years
SOUTHERN Co GAS | Maximum | Storage and transportation contracts
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			5 years
SOUTHERN Co GAS | Utility Plant in Service
Acquired Finite-Lived Intangible Assets [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate			2.90%	2.80%
SOUTHERN Co GAS | Predecessor
Finite-Lived Intangible Assets, Net [Abstract]
Accumulated Amortization	(34)			$ (34)
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
Amortization of intangible assets		$ 8			$ 18
SOUTHERN Co GAS | Predecessor | Minimum | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life				11 years
SOUTHERN Co GAS | Predecessor | Minimum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life				10 years
SOUTHERN Co GAS | Predecessor | Maximum | Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life				16 years
SOUTHERN Co GAS | Predecessor | Maximum | Trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life				28 years
SOUTHERN Co GAS | Predecessor | Utility Plant in Service
Acquired Finite-Lived Intangible Assets [Line Items]
Depreciation of cost of utility plant in service, composite straight-line rate					2.70%
SOUTHERN POWER CO
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life			19 years
Finite-Lived Intangible Assets, Net [Abstract]
Accumulated Amortization	(22)		$ (47)	$ (22)
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
2018			25
2019			25
2020			25
2021			25
2022			25
Amortization of intangible assets			25	10	$ 3
Gas Distribution Operations | SOUTHERN Co GAS
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	4,702		4,702	4,702
Gas Marketing Services | SOUTHERN Co GAS
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	1,265		1,265	$ 1,265
Storage and Fuels Reporting Unit | SOUTHERN Co GAS
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill						$ 14
Wholesale Gas Services | SOUTHERN Co GAS
Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract]
Amortization of intangible assets	$ 2		$ 32