Acquisitions - Narrative (Details) $ / shares in Units, MW in Thousands, $ in Millions			3 Months Ended		36 Months Ended
	Jul. 01, 2016 USD ($)	May 09, 2016 USD ($) $ / shares	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2020	Dec. 31, 2016 USD ($) MW
Business Acquisition [Line Items]
Goodwill			$ 6,251			$ 6,251
Revenues			5,771	$ 3,992
Consolidated net income attributable to Southern Company			658	489
Construction work in progress			9,465			8,977
Southern Company Gas [Member]
Business Acquisition [Line Items]
Goodwill			5,967			5,967
Revenues			1,560
Consolidated net income attributable to Southern Company			239
Construction work in progress			625			496
Southern Power [Member]
Business Acquisition [Line Items]
Goodwill			2			2
Revenues			450	315
Consolidated net income attributable to Southern Company			$ 70	$ 50
Number of projects to be placed in service			3
Total costs of construction			$ 401
Construction work in progress			328			$ 398
Southern Company Gas [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 8,000
Goodwill			$ 5,967
Southern Company Gas [Member] | Minimum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets			1 year
Southern Company Gas [Member] | Maximum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets			28 years
PowerSecure International, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price		$ 429
Goodwill			$ 282
Acquisition, share price (in dollars per share) | $ / shares		$ 18.75
PowerSecure International, Inc. [Member] | Minimum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets			1 year
PowerSecure International, Inc. [Member] | Maximum [Member]
Business Acquisition [Line Items]
Estimated lives of intangible assets			26 years
Series of Business Acquisitions [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Revenue of acquirees, actual			$ 4
Series of Construction Projects [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Construction work in progress			203
Series of Construction Projects [Member] | Minimum [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments			530
Series of Construction Projects [Member] | Maximum [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Estimated future construction payments			$ 590
Renewable Energy Systems Americas, Inc. [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Approx. nameplate capacity, wind (MW) | MW						3
Wind Generating Facility [Member] | Scenario, Forecast [Member] | Southern Power [Member]
Business Acquisition [Line Items]
Project qualification for production tax credits, percentage					100.00%
Production tax credits, project qualification period					10 years