Acquisitions and Dispositions - Narrative (Details) $ / shares in Units, MW in Thousands, $ in Millions			1 Months Ended		3 Months Ended			9 Months Ended		36 Months Ended
	Jul. 01, 2016 USD ($)	May 09, 2016 USD ($) $ / shares	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2020	Jan. 06, 2017	Dec. 31, 2016 USD ($) MW
Business Acquisition [Line Items]
Goodwill				$ 6,267		$ 6,267		$ 6,267				$ 6,251
Revenues						6,201	$ 6,264	17,403	$ 14,715
Consolidated net income attributable to Southern Company						1,069	1,139	347	2,251
Construction work in progress				8,026		8,026		8,026				8,977
SOUTHERN Co GAS
Business Acquisition [Line Items]
Goodwill				5,967		5,967		5,967				5,967
Revenues						565	543	2,841
Consolidated net income attributable to Southern Company						15	4	303
Construction work in progress				596		596		596				496
SOUTHERN POWER CO
Business Acquisition [Line Items]
Goodwill				2		2		2				2
Revenues						618	500	1,597	1,189
Consolidated net income attributable to Southern Company						124	176	276	315
Total costs of construction								494
Construction work in progress				425		425		425				$ 398
Repayments of debt				40
SOUTHERN Co GAS
Business Acquisition [Line Items]
Aggregate purchase price	$ 8,000
Goodwill				5,967		5,967		$ 5,967
SOUTHERN Co GAS | Minimum
Business Acquisition [Line Items]
Estimated lives of intangible assets								1 year
SOUTHERN Co GAS | Maximum
Business Acquisition [Line Items]
Estimated lives of intangible assets								28 years
PowerSecure International, Inc.
Business Acquisition [Line Items]
Aggregate purchase price		$ 429
Goodwill				284		284		$ 284
Acquisition, share price (in dollars per share) | $ / shares		$ 18.75
PowerSecure International, Inc. | Minimum
Business Acquisition [Line Items]
Estimated lives of intangible assets								1 year
PowerSecure International, Inc. | Maximum
Business Acquisition [Line Items]
Estimated lives of intangible assets								26 years
Bethel | SOUTHERN POWER CO
Business Acquisition [Line Items]
Ownership percentage by parent											100.00%
Series of Construction Projects | SOUTHERN POWER CO
Business Acquisition [Line Items]
Construction work in progress				122		122		$ 122
Series of Construction Projects | Minimum | SOUTHERN POWER CO
Business Acquisition [Line Items]
Estimated future construction payments				360		360		360
Series of Construction Projects | Maximum | SOUTHERN POWER CO
Business Acquisition [Line Items]
Estimated future construction payments				$ 415		415		415
Scenario, Forecast | Elizabethtown Gas and Elkton Gas
Business Acquisition [Line Items]
Aggregate purchase price					$ 1,700
Renewable Energy Systems Americas, Inc. | SOUTHERN POWER CO
Business Acquisition [Line Items]
Approximate nameplate capacity, wind (MW) | MW												3
Wind Generating Facility | Scenario, Forecast | SOUTHERN POWER CO
Business Acquisition [Line Items]
Project qualification for production tax credits, percentage										100.00%
Production tax credits, project qualification period										10 years
Operating Segments | SOUTHERN Co GAS
Business Acquisition [Line Items]
Revenues						607	586	3,000
Consolidated net income attributable to Southern Company						44		325
SOUTHERN Co GAS | Operating Segments
Business Acquisition [Line Items]
Revenues						565	543	2,841	543
Consolidated net income attributable to Southern Company						$ 15	$ 4	$ 303	$ 4
Subsequent Event | Southern Turner Renewable Energy, LLC [Member] | SOUTHERN POWER CO
Business Acquisition [Line Items]
Ownership percentage by parent			10.00%
Repayments of debt			$ 14