Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Concentration Risk [Line Items]
Operating revenues	$ 5,995	$ 6,159	$ 16,505	$ 18,158
Retail
Concentration Risk [Line Items]
Operating revenues	4,512	4,605	11,136	11,913
Wholesale
Concentration Risk [Line Items]
Operating revenues	625	698	1,667	1,937
Other
Concentration Risk [Line Items]
Operating revenues	163	165	492	495
Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues	445	433	2,169	2,299
Electric Utilities' Revenues
Concentration Risk [Line Items]
Operating revenues	5,363	5,509	13,448	14,456
Electric Utilities' Revenues | Retail
Concentration Risk [Line Items]
Operating revenues	4,512	4,605	11,136	11,913
Electric Utilities' Revenues | Wholesale
Concentration Risk [Line Items]
Operating revenues	625	698	1,667	1,937
Electric Utilities' Revenues | Other
Concentration Risk [Line Items]
Operating revenues	226	206	645	606
Southern Company Gas' Revenues
Concentration Risk [Line Items]
Operating revenues	498	492	2,661	2,861
Southern Company Gas' Revenues | Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues	445	438	2,169	2,276
Southern Company Gas' Revenues | Gas Distribution Operations | Revenues | Product Concentration Risk | Three Gas Distribution Operations
Concentration Risk [Line Items]
Operating revenues		7		245
Southern Company Gas' Revenues | Gas Marketing Services
Concentration Risk [Line Items]
Operating revenues	39	44	326	403
Southern Company Gas' Revenues | Gas Marketing Services | Revenues | Product Concentration Risk | Pivotal Home Solutions
Concentration Risk [Line Items]
Operating revenues				55
Southern Company Gas' Revenues | Other
Concentration Risk [Line Items]
Operating revenues	$ 14	$ 10	$ 166	$ 182