Merger, Acquisitions, and Dispositions - Acquisitions and Dispositions Table (Details) $ in Millions														6 Months Ended	12 Months Ended
	Jan. 26, 2018 MW	Jul. 31, 2017 MW	Jan. 06, 2017 MW	Dec. 21, 2016 MW	Dec. 01, 2016 MW	Nov. 16, 2016	Oct. 26, 2016 MW	Aug. 26, 2016 MW	Jul. 01, 2016 MW	Jun. 30, 2016 USD ($) MW	Apr. 07, 2016 MW	Mar. 04, 2016 MW	Feb. 11, 2016 MW	Dec. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Feb. 10, 2016	Dec. 31, 2015 USD ($)	Aug. 31, 2015 MW
Purchase Price Allocation
Goodwill														$ 6,251	$ 6,251	$ 6,268
Total purchase price														2,603	2,603
Acquisition Information, by Acquisition [Abstract]
Assets														109,697	109,697	111,005		$ 78,318
PowerSecure International, Inc.
Purchase Price Allocation
Current assets																172
Property, plant, and equipment																46
Goodwill																106
Intangible assets																284
Other assets																4
Current liabilities																(121)
Long-term debt, including current portion																48
Other liabilities																(14)
Total purchase price																429
SOUTHERN Co GAS
Purchase Price Allocation
Current assets																1,557
Property, plant, and equipment																10,108
Goodwill																5,967
Intangible assets																400
Regulatory assets																1,118
Other assets																229
Current liabilities																(2,201)
Other liabilities																(4,742)
Long-term debt																(4,261)
Noncontrolling interest																(174)
Total purchase price																8,001
Rutherford
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent							90.10%
Series of Business Acquisitions
Purchase Price Allocation
CWIP														2,354	2,354
Property, plant, and equipment														302	302
Intangible assets														128	128
Other assets														52	52
Accounts payable														(16)	(16)
Long-term debt														(217)	(217)
Total purchase price														2,345	2,345
SOUTHERN POWER CO
Purchase Price Allocation
Total purchase price																539
Acquisition Information, by Acquisition [Abstract]
Assets														15,169	15,169	$ 15,206
SOUTHERN POWER CO | Boulder 1
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW			276																100
Noncontrolling ownership percentage held by parent			100.00%			51.00%
Life output of plant			12 years			20 years
SOUTHERN POWER CO | Cactus Flats
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW		148
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent		100.00%
SOUTHERN POWER CO | Bethel
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW			276
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent			100.00%
Life output of plant			12 years
SOUTHERN POWER CO | 70SM1 8ME, LCC (Calipatria)
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW													20
Noncontrolling ownership percentage held by parent													100.00%				90.00%
Percentage of voting interests acquired																10.00%
Life output of plant													20 years
SOUTHERN POWER CO | East Pecos [Member]
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW												120
Noncontrolling ownership percentage held by parent												100.00%
Life output of plant												15 years
SOUTHERN POWER CO | Grant Plains
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW								147
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent								100.00%
Life output of plant								12 years
SOUTHERN POWER CO | Mankato
Acquisition Information, by Acquisition [Abstract]
Capacity of natural gas facility | MW							375
Noncontrolling ownership percentage held by parent							100.00%
Life output of plant							10 years
SOUTHERN POWER CO | Passadumkeag, LLC
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW										42
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent										100.00%
Life output of plant										15 years
SOUTHERN POWER CO | Rutherford
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW									74
Noncontrolling ownership percentage held by parent									90.00%				100.00%
Life output of plant									15 years
SOUTHERN POWER CO | Salt Fork
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW					174
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent					100.00%
SOUTHERN POWER CO | Series of Business Acquisitions
Business Acquisition [Line Items]
Contingent consideration														281	281
Purchase Price Allocation
CWIP																$ 534
Intangible assets																16
Other assets																5
Accounts payable																(16)
SOUTHERN POWER CO | Grant Wind, LLC
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW											151
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent											100.00%
Life output of plant											20 years
SOUTHERN POWER CO | Henrietta
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW									102
Noncontrolling ownership percentage held by parent									51.00%
Life output of plant									20 years
SOUTHERN POWER CO | Lamesa
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW									102
Noncontrolling ownership percentage held by parent									100.00%
Life output of plant									15 years
SOUTHERN POWER CO | Tyler Bluff
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW				125
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent				100.00%
Life output of plant				12 years
SOUTHERN POWER CO | Wake Wind
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW							257
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent							90.10%
Life output of plant							12 years
Turner Renewable Energy | 70SM1 8ME, LCC (Calipatria)
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent													10.00%
Turner Renewable Energy | Rutherford
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent									10.00%
Noncontrolling Interests | Series of Business Acquisitions
Business Acquisition [Line Items]
Contingent consideration														142	142
Purchase Price Allocation
Total purchase price														258	$ 258
Predecessor | SOUTHERN Co GAS
Purchase Price Allocation
Current assets										$ 1,474
Property, plant, and equipment										10,148
Goodwill										1,813
Intangible assets										101
Regulatory assets										679
Other assets										273
Current liabilities										(2,205)
Other liabilities										(4,600)
Long-term debt										(3,709)
Noncontrolling interest										(41)
Total purchase price										$ 3,933
Change in Basis
Current assets														83
Property, plant, and equipment														(40)
Other intangible assets														299
Goodwill														4,154
Regulatory assets														439
Other assets														(44)
Current liabilities														4
Other liabilities														(142)
Long-term debt														(552)
Contingently redeemable noncontrolling interest														(133)
Total purchase price/equity														4,068
Minimum | SOUTHERN POWER CO
Acquisition Information, by Acquisition [Abstract]
Life output of plant															10 years
Minimum | SOUTHERN POWER CO | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
Life output of plant		12 years
Maximum | SOUTHERN POWER CO
Acquisition Information, by Acquisition [Abstract]
Life output of plant															20 years
Maximum | SOUTHERN POWER CO | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
Life output of plant		15 years
Subsequent Event | SOUTHERN POWER CO | Gaskell West 1
Acquisition Information, by Acquisition [Abstract]
Power of solar polycrystalline silicon facility | MW	20
Life output of plant	20 years
Grant County | SOUTHERN POWER CO | Grant Plains
Acquisition Information, by Acquisition [Abstract]
Life output of plant								20 years
Donley County | SOUTHERN POWER CO | Salt Fork
Acquisition Information, by Acquisition [Abstract]
Life output of plant					14 years
Gray County | SOUTHERN POWER CO | Salt Fork
Acquisition Information, by Acquisition [Abstract]
Life output of plant					12 years
Senior Lien | SOUTHERN POWER CO
Acquisition Information, by Acquisition [Abstract]
Assets																442
Class B Membership Interest | SOUTHERN POWER CO | Cactus Flats
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent		100.00%
Class B Membership Interest | Subsequent Event | SOUTHERN POWER CO | Gaskell West 1
Acquisition Information, by Acquisition [Abstract]
Noncontrolling ownership percentage held by parent	100.00%
Acquisition Payable | SOUTHERN POWER CO | Series of Business Acquisitions
Business Acquisition [Line Items]
Contingent consideration														$ 461	$ 461
Payables | SOUTHERN POWER CO | Series of Business Acquisitions
Business Acquisition [Line Items]
Contingent consideration																$ 29
Allianz Risk Transfer (Bermuda) Ltd. | SOUTHERN POWER CO | Grant Plains
Acquisition Information, by Acquisition [Abstract]
Life output of plant								10 years