Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Total operating revenues	$ 6,159	$ 6,201	$ 18,158	$ 17,403
Retail
Revenue from External Customer [Line Items]
Total operating revenues	4,605	4,615	11,913	11,786
Electric Utilities' Revenues
Revenue from External Customer [Line Items]
Total operating revenues	5,509	5,523	14,456	14,239
Electric Utilities' Revenues | Retail
Revenue from External Customer [Line Items]
Total operating revenues	4,605	4,615	11,913	11,786
Electric Utilities' Revenues | Wholesale
Revenue from External Customer [Line Items]
Total operating revenues	693	718	1,923	1,867
Electric Utilities' Revenues | Other
Revenue from External Customer [Line Items]
Total operating revenues	211	190	620	586
Southern Company Gas' Revenues
Revenue from External Customer [Line Items]
Total operating revenues	492	565	2,861	2,841
Southern Company Gas' Revenues | Gas Distribution Operations
Revenue from External Customer [Line Items]
Total operating revenues	438	430	2,276	2,119
Southern Company Gas' Revenues | Gas Marketing Services
Revenue from External Customer [Line Items]
Total operating revenues	44	143	403	597
Southern Company Gas' Revenues | Other
Revenue from External Customer [Line Items]
Total operating revenues	$ 10	$ (8)	$ 182	$ 125