Merger, Acquisitions, and Dispositions - Textual (Details) $ / shares in Units, $ in Millions									1 Months Ended		3 Months Ended									6 Months Ended		12 Months Ended
		Jan. 26, 2018 MW	Mar. 31, 2017 USD ($)	Oct. 26, 2016 MW	Sep. 01, 2016 USD ($)	Aug. 26, 2016 MW	Jul. 01, 2016 USD ($) $ / shares MW shares	Feb. 11, 2016 MW	Sep. 30, 2016 USD ($) mi	May 31, 2016 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / shares MW shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares MW shares	Sep. 30, 2016 USD ($) mi	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) $ / shares MW shares	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / shares MW shares	Dec. 31, 2016 USD ($) $ / shares MW shares	Dec. 31, 2015 USD ($)	Oct. 31, 2016 USD ($)	Oct. 02, 2016	Jul. 31, 2016 MW	Feb. 10, 2016	Aug. 31, 2015 MW
Business Acquisition [Line Items]
Business Combination, Consideration Transferred																						$ 539
Revenue of acquiree																						15
Net income of acquiree																						17
Goodwill												$ 6,268				$ 6,251				$ 6,251		6,268	$ 6,251
Revenues												5,629	$ 6,201	$ 5,430	$ 5,771	5,181	$ 6,264	$ 4,459	$ 3,992			23,031	19,896	$ 17,489
Net income after dividends on preferred and preference stock												496	1,069	(1,381)	658	197	1,139	623	489			842	2,448	2,367
2018												95										95
Acquisitions payable												$ 5				$ 489				$ 489		5	489
Contributions from noncontrolling interests																						$ 79	$ 618	567
Common stock, shares authorized | shares												1,500,000,000				1,500,000,000				1,500,000,000		1,500,000,000	1,500,000,000
Common stock, par value per share (in dollars per share) | $ / shares												$ 5				$ 5				$ 5		$ 5	$ 5
Construction work in progress												$ 6,904				$ 8,977				$ 8,977		$ 6,904	$ 8,977
Tax Cuts And Jobs Act Of 2017, incomplete accounting, provisional income tax expense (benefit)												264
Payments to acquire equity method investments																						152	1,444	0
PowerSecure International, Inc.
Business Acquisition [Line Items]
Intangible assets												284										284
Goodwill												106										106
Business acquisition share price (in dollars per share) | $ / shares										$ 18.75
Business combination, consideration transferred, equity interests issued and issuable										$ 429
Current liabilities												121										121
SOUTHERN Co GAS
Business Acquisition [Line Items]
Intangible assets												400										400
Goodwill												5,967										5,967
Payments to acquire businesses							$ 8,000
Current liabilities												2,201										2,201
Common stock, shares authorized | shares							100,000,000
Preference stock, shares authorized | shares							10,000,000
Common stock, par value per share (in dollars per share) | $ / shares							$ 0.01
Rutherford
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent				90.10%
Series of Business Acquisitions
Business Acquisition [Line Items]
Intangible assets																128				128			128
SOUTHERN POWER CO
Business Acquisition [Line Items]
Payments to acquire businesses																						$ 2,300
Finite-lived intangible asset, useful life																						19 years
Revenues												478	618	529	450	389	500	373	315			$ 2,075	1,577	1,390
Net income after dividends on preferred and preference stock												795	124	82	70	23	176	89	$ 50
2018												25										25
Acquisitions payable												$ 5				$ 461				$ 461		5	461
Contributions from noncontrolling interests																						$ 79	$ 618	567
Common stock, shares authorized | shares												1,000,000				1,000,000				1,000,000		1,000,000	1,000,000
Common stock, par value per share (in dollars per share) | $ / shares												$ 0.01				$ 0.01				$ 0.01		$ 0.01	$ 0.01
Construction work in progress												$ 511				$ 398				$ 398		$ 511	$ 398
Capital contributions from parent company																						$ (2)	1,850	646
Tax Cuts And Jobs Act Of 2017, incomplete accounting, provisional income tax expense (benefit)												$ (743)
SOUTHERN POWER CO | Mankato Expansion [Member]
Business Acquisition [Line Items]
Capacity of natural gas facility | MW												345										345
SOUTHERN POWER CO | Desert Stateline Holdings, LLC
Business Acquisition [Line Items]
Power of solar polycrystalline silicon facility | MW																											299		299
Noncontrolling ownership percentage held by parent																			15.00%										51.00%
SOUTHERN POWER CO | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A
Business Acquisition [Line Items]
Contributions from noncontrolling interests																							51
SOUTHERN POWER CO | 70SM1 8ME, LCC (Calipatria)
Business Acquisition [Line Items]
Power of solar polycrystalline silicon facility | MW								20
Noncontrolling ownership percentage held by parent								100.00%																				90.00%
Life output of plant								20 years
SOUTHERN POWER CO | Rutherford
Business Acquisition [Line Items]
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 | Grant Plains
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent						100.00%
Life output of plant						12 years
Energy From Wind-Powered Generating Facilities | MW						147
SOUTHERN POWER CO | Mankato
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent				100.00%
Life output of plant				10 years
Capacity of natural gas facility | MW				375
SOUTHERN POWER CO | Series of Business Acquisitions
Business Acquisition [Line Items]
Intangible assets												$ 16										$ 16
2018																9				9			9
Contingent consideration																281				281			281
SOUTHERN Co GAS
Business Acquisition [Line Items]
Merger-related expenses																				41		0
Goodwill												5,967				5,967				5,967		5,967	$ 5,967
Revenues												1,079	565	716	1,560	1,109	543			1,652		3,920
Net income after dividends on preferred and preference stock												(60)	$ 15	$ 49	$ 239	$ 110	$ 4			$ 114		243
2018												$ 58										$ 58
Common stock, shares authorized | shares												100,000,000				100				100		100,000,000	100
Common stock, par value per share (in dollars per share) | $ / shares												$ 0.01				$ 10,000.00				$ 10,000.00		$ 0.01	$ 10,000.00
Construction work in progress												$ 491				$ 496				$ 496		$ 491	$ 496
Capital contributions from parent company																				1,094		117
Payments to acquire equity method investments																				1,444		145
SOUTHERN Co GAS | Southern Company
Business Acquisition [Line Items]
Business acquisition share price (in dollars per share) | $ / shares							$ 66
SOUTHERN Co GAS | SOUTHERN Co GAS
Business Acquisition [Line Items]
Goodwill												$ 6,000										6,000
Shares, authorized | shares							110,000,000
SOUTHERN Co GAS | Southern Natural Gas Company, LLC
Business Acquisition [Line Items]
Pipeline infrastructure | mi									7,000								7,000
Noncontrolling Interests | Series of Business Acquisitions
Business Acquisition [Line Items]
Contingent consideration																$ 142				$ 142			$ 142
Invenergy | Rutherford
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent				9.90%
Turner Renewable Energy | 70SM1 8ME, LCC (Calipatria)
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent								10.00%
Turner Renewable Energy | Rutherford
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent							10.00%
Southern Power, Turner Renewable Energy, SunPower, and Invenergy
Business Acquisition [Line Items]
Payments to acquire businesses																						$ 2,600
Elizabethtown Gas | SOUTHERN Co GAS
Business Acquisition [Line Items]
Business combination, regulatory approval requirements, base rate case fling period																						3 years
Elkton Gas | SOUTHERN Co GAS
Business Acquisition [Line Items]
Business combination, regulatory approval requirements, base rate case fling period																						2 years
Class A Membership Interest | SOUTHERN POWER CO | Desert Stateline Holdings, LLC
Business Acquisition [Line Items]
Percentage of entitled cash distributions																66.00%				66.00%			66.00%
Class A Membership Interest | SOUTHERN POWER CO | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent												100.00%										100.00%
Percentage of entitled cash distributions												51.00%										51.00%
Class B Membership Interest | First Solar and Recurrent Energy | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent												100.00%										100.00%
Class B Membership Interest | First Solar | Desert Stateline Holdings, LLC
Business Acquisition [Line Items]
Percentage of entitled cash distributions																34.00%				34.00%			34.00%
Class B Membership Interest | First Solar | Lost Hills Blackwell, North Star, Tranquillity, Desert Stateline, Roserock, Garland and Garland A
Business Acquisition [Line Items]
Percentage of entitled cash distributions												49.00%										49.00%
Minimum | PowerSecure International, Inc.
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life																						1 year
Minimum | SOUTHERN Co GAS
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life																						1 year
Minimum | SOUTHERN POWER CO
Business Acquisition [Line Items]
Life output of plant																							10 years
Minimum | SOUTHERN POWER CO | Series of Business Acquisitions
Business Acquisition [Line Items]
Estimated Future Construction Payments												$ 385										$ 385
Maximum | PowerSecure International, Inc.
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life																						26 years
Maximum | SOUTHERN Co GAS
Business Acquisition [Line Items]
Finite-lived intangible asset, useful life																						28 years
Maximum | SOUTHERN POWER CO
Business Acquisition [Line Items]
Life output of plant																							20 years
Maximum | SOUTHERN POWER CO | Series of Business Acquisitions
Business Acquisition [Line Items]
Estimated Future Construction Payments												$ 430										$ 430
Successor | SOUTHERN Co GAS
Business Acquisition [Line Items]
Goodwill																$ 30
Successor | Elizabethtown Gas and Elkton Gas
Business Acquisition [Line Items]
Business combination, regulatory approval requirements, required rate credit payments to customers																				$ 18
Predecessor | SOUTHERN Co GAS
Business Acquisition [Line Items]
Goodwill																				$ 4,154
Intangible assets																		101			$ 101
Goodwill																		1,813			1,813
Current liabilities																		2,205			2,205
Predecessor | SOUTHERN Co GAS
Business Acquisition [Line Items]
Merger-related expenses																					56			44
Revenues																		571	$ 1,334		1,905			3,941
Net income after dividends on preferred and preference stock																		$ (51)	$ 182		131			353
Payments to acquire equity method investments																					$ 14			12
Subsequent Event | SOUTHERN POWER CO | Gaskell West 1
Business Acquisition [Line Items]
Power of solar polycrystalline silicon facility | MW		20
Life output of plant		20 years
Subsequent Event | Class B Membership Interest | SOUTHERN POWER CO | Gaskell West 1
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent		100.00%
SNG | SOUTHERN Co GAS
Business Acquisition [Line Items]
Capital contributions from parent company					$ 1,050
Proceeds from contributions from affiliates									$ 360
Equity method investment, difference between carrying amount and underlying equity									$ 700								$ 700
Tax Cuts And Jobs Act Of 2017, incomplete accounting, provisional income tax expense (benefit)																						$ 104
Southstar | SOUTHERN Co GAS
Business Acquisition [Line Items]
Noncontrolling ownership percentage held by parent																										85.00%
Southern Natural Gas Company, LLC | SOUTHERN Co GAS
Business Acquisition [Line Items]
Ownership percentage, equity method investment									50.00%								50.00%
Equity method investment, aggregate cost									$ 1,400								$ 1,400
Payments to acquire equity method investments			$ 50
Southstar | SOUTHERN Co GAS
Business Acquisition [Line Items]
Ownership percentage of noncontrolling interest									85.00%								85.00%
Agreement to purchase remaining interest																									$ 160
Southstar | Piedmont
Business Acquisition [Line Items]
Ownership percentage of noncontrolling interest									15.00%								15.00%
Siemens Wind Power, Inc. And Vestas-American Wind Technology, Inc. [Member] | SOUTHERN POWER CO
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW												900										900
Renewable Energy Systems Americas, Inc. [Member] | SOUTHERN POWER CO
Business Acquisition [Line Items]
Energy From Wind-Powered Generating Facilities | MW																3,000				3,000			3,000
Allianz Risk Transfer (Bermuda) Ltd. | SOUTHERN POWER CO | Grant Plains
Business Acquisition [Line Items]
Life output of plant						10 years
Wind Generating Facility | SOUTHERN POWER CO
Business Acquisition [Line Items]
Project Qualification for Production Tax Credits, Percentage																						80.00%	100.00%
Grant County | SOUTHERN POWER CO | Grant Plains
Business Acquisition [Line Items]
Life output of plant						20 years
Noncontrolling Interests
Business Acquisition [Line Items]
Contributions from noncontrolling interests																						$ 79	$ 618	567
Noncontrolling Interests | SOUTHERN POWER CO
Business Acquisition [Line Items]
Contributions from noncontrolling interests	[1]																					79	$ 618	$ 567
Subsidiaries | Scenario, Forecast
Business Acquisition [Line Items]
Proceeds from sale of oil and gas property and equipment											$ 1,700
Construction in Progress [Member] | Series of Business Acquisitions
Business Acquisition [Line Items]
Construction and Development Costs																						$ 188

[1] Excludes redeemable noncontrolling interests. See Note 10 to the financial statements under "Noncontrolling Interests" for additional information.