Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 4,565	$ 4,585	$ 8,948	$ 8,716
Southern Company Gas' Revenues	730	716	2,369	2,276
Retail
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	3,740	3,777	7,308	7,171
Wholesale
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	611	618	1,230	1,149
Other
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	214	190	410	396
Gas Distribution Operations
Revenue from External Customer [Line Items]
Southern Company Gas' Revenues	638	557	1,838	1,689
Gas Marketing Services
Revenue from External Customer [Line Items]
Southern Company Gas' Revenues	89	166	359	454
Other
Revenue from External Customer [Line Items]
Southern Company Gas' Revenues	$ 3	$ (7)	$ 172	$ 133