Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Concentration Risk [Line Items]
Operating revenues	$ 5,412	$ 6,372
Retail
Concentration Risk [Line Items]
Operating revenues	3,084	3,568
Wholesale
Concentration Risk [Line Items]
Operating revenues	499	623
Other
Concentration Risk [Line Items]
Operating revenues	168	161
Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues	1,163	1,224
Electric Utilities' Revenues
Concentration Risk [Line Items]
Operating revenues	3,795	4,382
Electric Utilities' Revenues | Retail
Concentration Risk [Line Items]
Operating revenues	3,084	3,568
Electric Utilities' Revenues | Wholesale
Concentration Risk [Line Items]
Operating revenues	499	623
Electric Utilities' Revenues | Other
Concentration Risk [Line Items]
Operating revenues	212	191
Southern Company Gas' Revenues
Concentration Risk [Line Items]
Operating revenues	1,474	1,639
Southern Company Gas' Revenues | Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues	1,161	1,200
Southern Company Gas' Revenues | Gas Distribution Operations | Revenues | Product Concentration Risk | Three Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues		167
Southern Company Gas' Revenues | Gas Marketing Services
Concentration Risk [Line Items]
Operating revenues	229	271
Southern Company Gas' Revenues | Gas Marketing Services | Revenues | Product Concentration Risk | Pivotal Home Solutions
Concentration Risk [Line Items]
Operating revenues		32
Southern Company Gas' Revenues | Other
Concentration Risk [Line Items]
Operating revenues	$ 84	$ 168